KYIV, Ukraine—For Pavel Kulazhanka, just like many Belarusian and Russian fighters who have joined Ukraine’s military in recent weeks, the path to freedom at home runs through defeating the Russian army here first.

Hundreds of volunteers from Belarus have joined a dedicated Belarusian battalion and other formations of Ukrainian armed forces since Russian President Vladimir Putin triggered the invasion on Feb. 24, with more than a 1,000 others still awaiting vetting and training, Belarusian opposition leaders say.

Under Ukrainian law, foreigners who serve in its armed forces are eligible for Ukrainian citizenship.

Freedom for Russia, a separate unit staffed by Russian citizens, including prisoners of war who have switched sides, is also preparing to deploy to the front lines. It is backed by some prominent Russian opposition leaders in exile, who say that the presence of Russian soldiers helping defend Ukraine and prevent atrocities would go a long way toward healing the rift between the two peoples in the future.

To the Belarusians, many of whom fled the repression unleashed by President Alexander Lukashenko after the fraud-marred presidential election of 2020, the fight in Ukraine is a steppingstone to liberating their own homeland. While Mr. Lukashenko stopped short of sending the Belarusian army to Ukraine, he has allowed Russia to use Belarus as a staging ground for the invasion, with tens of thousands of Russian troops deployed there and Russian warplanes taking off from Belarusian bases.

Police in Minsk, Belarus, during the 2020 protests against President Alexander Lukashenko.

Photo: Jedrzej Nowicki for The Wall Street Journal

“Without an independent Ukraine, there won’t be an independent Belarus,” said Mr. Kulazhanka, who arrived in Kyiv several weeks ago and is now a soldier in the Belarusian-run Kastus Kalinouski Battalion. “This is the first stop; the second stop is going to be Belarus.”

The battalion is named after a leader of the 1863 uprising against czarist Russia in what is now Belarus, Lithuania, Poland and Ukraine. It has about 200 members so far, from activists and bloggers with no combat experience to veterans of the 2014-2015 war against Russian-backed forces in eastern Ukraine’s Donbas region. The battalion has already engaged in battles to defend Kyiv, which Russian forces failed to seize after five weeks of fierce battles.

There is another Belarusian battalion being formed, Pahonia, as well as Belarusian units within Ukrainian territorial defense forces in the cities of Odessa and Lutsk, said Franak Viacorka, an adviser to Belarusian opposition leader Sviatlana Tsikhanouskaya, who stood against Mr. Lukashenko in the 2020 presidential election and is exiled in Lithuania. In addition, Belarusian activists have been working to sabotage railways used by Russian forces to ship troops and supplies to the Kyiv front.

“The very existence of Belarus is under threat now. When Putin says that Ukrainian statehood shouldn’t exist, he means the same about Belarus,” Mr. Viacorka said. “The fact that Russia’s army renounced the idea of seizing Kyiv is also the merit of those Belarusians who made the siege of Kyiv too costly and made the Russian rear insecure.”

People protested against Mr. Lukashenko in Minsk’s Independence Square in 2020.

Photo: Jedrzej Nowicki for The Wall Street Journal

Belarusians fighting in the ranks of the Ukrainian armed forces today will be building the army of a new Belarus once Mr. Lukashenko’s regime collapses, he added.

Belarus’s population, unlike Russia’s, is deeply opposed to the war in Ukraine, said U.S. Special Envoy for Belarus Julie Fisher.

“There is a real disbelief—by those inside and outside Belarus—that Lukashenko would allow Belarusian territory to be used like this,” she said.

Shortly after Russia started the war, Ukrainian President Volodymyr Zelensky invited volunteers from all over the world to come help fight for Ukraine’s independence in the newly created International Legion, a unit created along the lines of the International Brigades that fought for the Spanish republic in the 1930s civil war there. The legion’s recruits include many citizens of Western nations, including former U.S. and British military veterans.

Pahonia is part of this International Legion but the Kastus Kalinouski battalion has been incorporated into regular Ukrainian forces under its own, Belarusian, commander. Ukrainian officers usually command units of the International Legion. Three members of the battalion have been killed in combat so far, according to Mr. Kulazhanka.

While the Belarusian opposition has long been inspired by Ukraine’s resistance to Russia and attempts to impose authoritarian rule, the arrival of Russians willing to fight against their own compatriots is relatively new. The uniforms of the Freedom for Russia unit have the white-blue-white patch, in the colors of a new Russian flag favored by some opponents of Mr. Putin.

Wearing black balaclavas, three men addressed a press conference Tuesday in Kyiv, explaining how they had joined the unit. Each of them said he had deployed to Ukraine with the Russian military but experienced second thoughts after being taken prisoner.

“It turned out that we were tricked into this war,” said one of the men, who said he had been a sergeant with the Russian special forces. “This legion has been created to fight against Putin’s regime.”

They declined to provide evidence of previous Russian service and said they had fought on the Ukrainian side but didn’t share details about units or battles in which they had participated. They also declined to say how the Ukrainian military used them or how many members were in their group. They said they had incorporated Belarusian soldiers into the legion.

Former Russian lawmaker Ilya Ponomarev, who was the only member of the Russian parliament to vote against the annexation of Ukraine’s Crimea peninsula in 2014 and now lives in Kyiv, said that Russian soldiers willing to fight for Ukraine today were akin to anti-Nazi Germans joining allied forces in World War II. He compared them to German statesman Willy Brandt, who fought against fellow Germans in Norway and then, after the defeat of Nazism, was elected West Germany’s federal chancellor.

Konstiantyn Sunzyk fled to Ukraine after Belarusian authorities issued a second warrant for his arrest in 2020.

“I know thousands and thousands of people in Russia who desire to fight in this war against Putin’s army. They don’t want to kill fellow Russians but they want to kill Putinists, who have become just the same as the fascists of 80 years ago,” Mr. Ponomarev added.

Marat Gelman, a Russian art collector and opposition politician who once advised Mr. Putin and served as deputy chief of Russia’s main state TV channel, said that while Russian volunteers fighting for Ukraine today may not be a significant military force, their political importance can’t be overstated.

“They represent the hope that, once the war is over, at least some conversation will become possible between us,” he said. “Once it becomes known how the citizens of Russia helped Ukraine repel this aggressor, it will facilitate dialogue between Russians and Ukrainians in the future. And these people could become intermediaries.”

In Belarus, Mr. Lukashenko’s bloody suppression of protests over the 2020 election, which Ms. Tsikhanouskaya says she won, has brought a massive increase in influence by Russia, which until recently didn’t have military bases on Belarusian soil.

Mr. Kulazhanka watched the crisis at home unfold from New York, where he worked as a bouncer and mixed martial arts trainer. Before moving to the U.S., Mr. Kulazhanka had served for seven years in Belarus’s special forces, and he began sharing his experience with protesters to help them stand up to riot police. He said he assisted in acts of resistance against the security forces from afar, including a drone attack on a Belarusian riot police base in Minsk.

Serhiy Bespalov fled Belarus in 2020 after several of his friends were detained for their role in street protests.

In New York when the invasion of Ukraine began, Mr. Kulazhanka headed straight to the Ukrainian consulate there to sign up. After weeks of waiting he hadn’t heard back and flew to Warsaw on his own initiative, going to a recruitment center there. From Poland, he traveled to Ukraine with other new recruits and caught a train to Kyiv.

A fellow soldier, Konstiantyn Sunzyk, fled to Ukraine after Belarusian authorities issued a second warrant for his arrest in 2020. The 31-year-old had worked on the campaign of Belarusian opposition leader Viktar Babaryka, who was imprisoned before the election and disqualified from running. From Ukraine, Mr. Sunzyk sought to continue raising awareness of the repression in Belarus despite fading international attention.

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Serhiy Bespalov also fled Belarus in 2020 after several of his friends were detained for their role in street protests. “I was pretty likely to be a target,” he said. The journalist and blogger, who runs several opposition Telegram channels and has no prior military experience, was in Kyiv when Russia invaded on Feb. 24.

In the early chaos of the invasion, Ukrainian forces detained Mr. Bespalov, suspecting him of being a Russian saboteur. He managed to prove he was on their side by showing them his Telegram channels and the tattoo on the back of his calf, which says “Stop Luka”—a reference to Mr. Lukashenko.

With a large online following, Mr. Bespalov is also part of the effort to draw new recruits to the battalion.

“As soon as we liberate Ukraine, we will go to liberate Minsk,” he said.

—Brett Forrest and Dan Michaels contributed to this article.

Write to Isabel Coles at isabel.coles@wsj.com and Yaroslav Trofimov at yaroslav.trofimov@wsj.com