Protesters clashed with riot police in the Ukrainian capital after tough anti-protest legislation was rushed through parliament last week.
Opposition politicians have said the new laws pave the way for a police state.
Demonstrators attacked a police cordon with sticks and tried to overturn a bus blocking their way to the parliament building.
Despite appeals from opposition leaders not to resort to violence, and a personal intervention from boxer-turned-politician Vitaly Klitschko, protesters threw smoke bombs and hurled fireworks and other objects at police.
The Interior Ministry said 30 police were hurt, including more than ten admitted to hospital and four were in a serious condition.
As tensions continued into the night, police used water cannon against demonstrators gathered near the parliament building and the heavily protected government headquarters.
Earlier, some distance away from the clashes, up to 100,000 Ukrainians massed on Kiev's Independence Square in defiance of the sweeping new laws, which ban rallies and which Washington and other Western capitals have denounced as undemocratic.
The White House said it was "deeply concerned" about the violence in Kiev and urged both sides to "de-escalate the situation."
"The increasing tension in Ukraine is a direct consequence of the government failing to acknowledge the legitimate grievances of its people," National Security Council spokeswoman Caitlin Hayden said in a statement.
"Instead, it has moved to weaken the foundations of Ukraine's democracy by criminalising peaceful protest and stripping civil society and political opponents of key democratic protections under the law."
Urging the Ukraine government to repeal the "anti-democraticlegislation," withdraw riot police from downtown Kiev and begina dialogue with the opposition, Ms Hayden added, that "the US will continue to consider additional steps - including sanctions – in response to the use of violence."
The rally, the biggest of the new year, was the latest in a cycle of public protests in the former Soviet republic since President Viktor Yanukovych made a policy U-turn in November away from the European Union towards Russia.
Several big protests in December attracted hundreds of thousands of people, while thousands maintained a vigil in a Kiev square demanding Mr Yanukovych resign.
Since the new year demonstrations have become smaller, but hundreds of people are still camping in the square and 50,000 turned out a week ago.