KIEV, Ukraine — The Ukrainian government used telephone technology to pinpoint the locations of cellphones in use near clashes between riot police officers and protesters early on Tuesday, illustrating that techniques that can be used to target commercial information can serve law enforcement as well.
People near the fighting between riot police and protesters received a text message shortly after midnight saying “Dear subscriber, you are registered as a participant in a mass disturbance.”
The phrasing echoed language in a new law making participation in a protest deemed violent a crime punishable by imprisonment. The law took effect on Tuesday.
This law and a package of other legislation passed by pro-government political parties in Parliament appeared modeled on rules in neighboring Russia, which has pioneered the coordination of legislation tightening rules on free speech and public assembly with technological capabilities.

In the civil unrest in the Ukrainian capital, Kiev, though, the text message seemed to have little effect. Three hours after it was sent, the riot police pushed past barricades of burned buses on Hrushevskoho Street near Parliament but were nonetheless met by a crowd of protesters in ski masks and helmets carrying sticks and ready to fight.
The police fired plastic bullets from shotguns and threw stun grenades. They pressed as far as a cobblestone-throwing catapult built by protesters the day before and dismantled it before retreating again.
In another tactic apparently being ramped up on Tuesday to pressure protesters, young men in sweatshirts carrying sticks roamed side streets near the central square, beating protesters and breaking shop windows, including the window of a bar on Khreshchatyk, the main street.
The opposition leaders had said they believed these people to be soccer hooligans and unemployed men bused into the capital by the government to provide a proxy force of street muscle to intimidate protesters and darken the image of the movement by inciting violence.

Witnesses reported on social media that as this unfolded around 4 a.m., the police had all but disappeared from the streets in downtown Kiev, apart from the riot forces confronting protesters and guarding the Parliament building.
“Disorders should not be allowed to happen,” Vitali Klitschko, a former heavyweight boxing champion and the 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 on Tuesday morning, opposition activists, wearing ski masks and wielding baseball bats, detained a dozen or so of these rival young men and marched them back to one of several buildings occupied by protesters.
Opposition activists questioned them in an exchange broadcast on an opposition-controlled television station. Several of the young men said they were promised about $25 to create disturbances near the square but did not explain clearly who had hired them.
An activist who has been prominent in the movement, Ihor Lutsenko, went missing Tuesday after unknown men forced him into a car in the parking lot of a hospital, according to a Facebook post by Mr. Lutsenko’s wife.
The protests began in November after President Viktor F. Yanukovich declined to sign a free-trade agreement with the European Union, negotiating a financial aid deal with Russia instead. A movement that seemed to be fading was reinvigorated by opposition to new laws against public assembly passed last week.
Oksana Lyachynska contributed reporting.