MOSCOW—A Belarusian activist who headed a group in Ukraine that helps Belarusians escape repression in their homeland was found dead a day after disappearing near his home in Kyiv, an incident that comes amid an intensifying crackdown on Belarusian activists.

Ukrainian authorities said Tuesday they were investigating the death of Vitaly Shyshov, who was found hanged in a park not far from his residence, as a homicide. All scenarios would be investigated, including the possibility of murder disguised as suicide, police said.

The incident comes as Belarusian President Alexander Lukashenko has stepped up a campaign to suppress any form of opposition to his nearly 30-year rule. A year after declaring victory in a presidential election that local activists and Western leaders widely believe was rigged, Mr. Lukashenko has launched raids on opponents’ homes, banned independent media outlets, allowed security personnel to use live ammunition against protesters, and jailed or exiled key opponents.

Mr. Shyshov, 26 years old, was forced to move to Ukraine in the fall of 2020 after participating in the mass protests that swept Belarus following the presidential vote, representatives of Belarusian House, the group he led, said in a statement on its Telegram messaging channel. As head of the humanitarian organization, he helped other Belarusians to relocate abroad, organized actions against Mr. Lukashenko’s regime, and appealed to government agencies to promote bills to assist Belarusians, the group said.

Ukrainian law-enforcement authorities had warned Mr. Shyshov that he was being watched, the group said.

The site where Mr. Shyshov was found dead in Kyiv.

The site where Mr. Shyshov was found dead in Kyiv.

Photo: gleb garanich/Reuters

“We were repeatedly warned by both local sources and our people in the Republic of Belarus about all kinds of provocations, including kidnapping and liquidation,” the group said. “Vitaly treated these warnings stoically and with humor.”

Surveillance cameras show Mr. Shyshov leaving his house and going for a run at 9 a.m. Kyiv time on Monday, representatives of Belarusian House said on Telegram. He was expected to return at around 10 a.m. but never showed up, Mr. Shyshov’s colleagues said.

Police said no personal belongings were found at the scene where Mr. Shyshov’s body was discovered.

Neither Mr. Lukashenko’s office nor Belarus’s Foreign Ministry immediately responded to requests for comment on Mr. Shyshov’s death.

Weekly mass rallies, calls for a national strike and sanctions imposed by Western nations have only strengthened Mr. Lukashenko’s resolve to remain in office and emboldened his actions.

In May, he scrambled a jet fighter to force a Ryanair PLC commercial aircraft to land in Minsk, where a Belarusian journalist and opposition activist, Roman Protasevich, was detained.

On Sunday, Belarusian Olympic athlete Krystsina Tsimanouskaya resisted her team’s attempts to send her home from Tokyo against her will in the middle of a public dispute and subsequently received a humanitarian visa to travel to Poland. Ms. Tsimanouskaya told The Wall Street Journal on Tuesday that she would never return home for fear for her life and was looking to continue her sporting career in another country. Her husband confirmed that he had fled to Ukraine. Representatives of Belarusian House said they hadn’t aided his escape.

Belarusian opposition leader Svetlana Tikhanovskaya, who was forced to flee to neighboring Lithuania shortly after last year’s presidential vote—in which she claimed victory—lamented Mr. Shyshov’s fate.

She and other activists have said that incidents like those involving Messrs. Shyshov and Protasevich and Ms. Tsimanouskaya have raised fears among Belarus’s expatriate dissident community that their lives could still be in grave danger even outside the country.

“Belarusians cannot be safe even abroad, as long as there are those who are trying to take revenge on them and hide the truth, getting rid of witnesses,” she said on Telegram on Tuesday.

Belarusian sprinter Krystsina Tsimanouskaya—who, fearing arrest, resisted her team’s attempt to send her home—is safe with Japanese authorities, the International Olympic Committee says. The situation tests the IOC’s aim to run a politics-free Games. Photo: PHOTO: Issei Kato/Reuters The Wall Street Journal Interactive Edition

—Valentina Ochirova contributed to this article.

Write to Ann M. Simmons at ann.simmons@wsj.com