KIEV, Ukraine —

Volodymyr Danyluk was a Soviet army veteran who joined demonstrations against Ukraine’s government last year. He was 55 years old, separated from his wife and mostly out of contact with his family, who saw him on live television during a winter of protests.

Then came the authorities’ crackdown last month in Kiev, the capital. The riot police and demonstrators clashed, scores of people were killed and the government of President Viktor Yanukovych fell. Danyluk disappeared from sight.

In the weeks since, Ukraine’s interim authorities have allowed opposition members to search prisons, morgues and hospitals for their missing. There has been no sign of Danyluk — or of more than 250 other Ukrainians.

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After a season of political upheaval here, a gnawing worry persists: What happened to Ukrainians who seemingly vanished in their revolution’s fast-moving tides? Were Danyluk and the others victims of state repression and criminal activity by the police, or had some of them just drifted back to quiet lives?

“Our mom is worried and calling me all the time,” said Danyluk’s sister, Galyna Onyshchuk, crying.

In all, 661 people have been reported missing since protests began last December, according to Euromaidan SOS, a volunteer group leading efforts to find those who have disappeared. The fates of 272 of them remained unknown late last week.

Many people were found in prison cells or hospitals, or resurfaced on their own, said Vitaliy Selyk, a Euromaidan SOS coordinator. In some cases, there were breakdowns in communications, including people who lost cellphones or ran out of credit on SIM cards, he said.

But beneath that hope lay the grim concern that many Ukrainians may have disappeared after being seized by the Berkut riot-police unit, by pro-Russian provocateurs or by unofficial forces that worked to keep Yanukovych in power.

This fear, cited almost universally by the opposition, is rooted in two particular cases.

The first was the killing of Yuriy Verbytskyi, a seismologist and an opposition activist, who was found dead in January in the forest near Boryspil after being abducted from a Kiev hospital.

A fellow abductee who survived, Igor Lutsenko, told journalists that their captors spoke Russian, beat them, interrogated them about their activities in the opposition and generally behaved like police officers.

The second case was the abduction of Dmytro Bulatov, an organizer of AutoMaidan, a mobile opposition movement that includes drivers who ferried demonstrators to protests.

Bulatov disappeared in late January. He turned up a week later, bloodied and bearing signs of torture that he said he received at the hands of people he believed were members of Russia’s special services.

Ukraine’s Interior Ministry, then under the control of Yanukovych, suggested that Bulatov had faked his own kidnapping as an anti-government provocation.

That assertion undercut the public’s confidence that the authorities would dedicate themselves to solving missing-person cases that might point to a government role or official complicity.

Abuses by the ministry’s riot police have been well established, including the taunting humiliation of Mihailo Gavryluk, a farmer from western Ukraine who, upon being arrested last month, was forced to stand outside naked in the wintry cold while masked and hooded police officers posed for photographs with him.

Part of this episode was posted in a video on YouTube. In it, one officer slaps him and gives him a stern kick.

Gavryluk said the Berkut officers who arrested him tore off his clothes. “They had fun,” he said, darkly.

But he noted that his luck soon turned better.

The Berkut police were busy, and often left detainees at police stations or hospitals scattered around the city. Gavryluk said he was taken to a hospital where veterans who fought in the Soviet war in Afghanistan and were loyal to the opposition were active.

The veterans quickly spirited him back to the square, he said. When the police returned to pick him up for prosecution, he was gone.

Another former detainee, Andriy Babyn, described a law-enforcement system overwhelmed by the large number of people arrested as Ukrainians turned out to fight. “There was practically a war going on,” he said.

One result, Babyn said, was that some detainees ended up at police stations or jails where officers were either neutral or sympathetic to the opposition.

More than two weeks since the last clashes, many people remain missing, even after thorough checks at hospitals and prisons. Rumors are rampant.

One macabre story, common in Independence Square, is that 50 opposition members being treated in a hastily organized medical-aid station in the Trade Union building were killed and burned beyond recognition by the fire that gutted the building.

This tale is largely false, people involved in the searches said. A few people did die in the blaze, they said, but the number of victims was six or fewer.

Among those lost in the fire was Volodymyr Topij, 59, from Vyshnya, for whom at least one missing-person circular was still posted in the opposition encampment. Topij’s remains were identified last week. His body was escorted home for burial.

A more sinister whisper on the street is that the authorities, to mask widespread police crimes, arranged for the cremation of more than 100 bodies of those they killed.