People tracked by cellphone to be near near fighting between riot police and protesters were texted, "Dear subscriber, you are registered as a participant in a mass disturbance," soon after midnight, after a new law making participation in a protest a violent crime punishable by a prison term, the New York Times reported.
Three hours after the text was sent, riot police pushed past makeshift barricades of burned vehicles in Kiev near the Parliament and were met by protesters wearing in ski masks and helmets, and carrying sticks, the Times said.
Police fired plastic bullets from shotguns and lobbed stun grenades into the crowd as they advanced to a rock-throwing catapult and dismantled it before retreating again.
A package of laws that restrict speech and protest were approved by the pro-government political parties in the Ukrainian Parliament were modeled on laws Russia passed a year ago. Russia coordinated legislation that restricted free speech and public assembly with technological capabilities.
The Times also reported men brandishing sticks roamed side streets near Kiev's central square, beating protesters and breaking shop windows.
Previously, opposition leaders said they thought these roving gangs were soccer hooligans and unemployed men transported into the capital by the government to provide a show of force to intimidate protesters and foul the image of the movement by inciting violence.
"Disorders should not be allowed to happen," Vitali Klitschko, leader of the political party Punch, wrote of the thugs on his Twitter account. "This is a plan of authorities to introduce a state of emergency."
Early Tuesday, opposition activists detained several of the hooligans and questioned them in a session broadcast on an opposition-controlled television station, the Times said. Several of the detainees said they were promised about $25 to create disturbances near the square but did not explain who hired them.
Also Tuesday, opposition activist Ihor Lutsenko disappeared after he was forced into a car, a Facebook post by Lutsenko's wife said.
The protests began in November after President Viktor Yanukovych decided not to sign a free-trade agreement with the European Union, saying it would hurt Ukraine. He later negotiated a financial aid deal with Russia.
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