In near apocalyptic scenes close to parliament, several police buses and vehicles were torched by the protesters who hurled stones and Molotov cocktails at the ranks of the security forces. Police responded with tear gas, stun grenades and rubber bullets.
The clashes, the worst in Kiev in recent times, further raised the stakes in the almost two-month standoff between the opposition and Yanukovych which has seen protesters seize control of the main city square in Kiev and municipal buildings.
The White House urged an end to the violence, with National Security Council spokeswoman Caitlin Hayden saying that Washington was deeply concerned and urging "all sides to immediately de-escalate the situation."
Yanukovych, in his first response to the violence, ordered the setting up of a commission that would hold talks with the opposition but it was unclear if this could in any way help ease the crisis.
Ukrainian police used tear gas, stun grenades and water cannon in a bid to disperse the hundreds of people who sought to storm police cordons near the Verkhovna Rada parliament and close to the stadium of the legendary Dynamo Kiev football club in central Kiev, witnesses and AFP correspondents said.
In the most violent scenes since the start of the protests in November, demonstrators set five buses and two trucks on fire while the air filled with the stench of tear gas and thick with the smoke from the stun grenades. Medical workers said police had also used rubber bullets.
Their faces covered by scarves or ski masks, many of the protesters wielded sticks or even chains. They were confronted by helmeted riot police equipped with shields who at times adopted a "tortoise" formation to protect themselves from falling projectiles.
Health officials said 24 people were injured and three were hospitalised, while police said more than 70 officers had been hurt. The interior ministry said four people had already been arrested and charged with mass rioting.
Kiev riot police said the protesters had captured a security services officer and brutally beat him up. He was then rescued but found seriously injured and in a "state of shock".
'Concerned' Yanukovych meets Klitschko
Opposition leaders including former boxing champion Vitali Klitschko and Arseniy Yatsenyuk called on the protesters to refrain from using force but their calls were ignored by the more radically-minded protesters.
Amid the chaos of the clashes, Klitschko was sprayed with powder from a fire extinguisher, leaving his eyes irritated and face and clothes covered in white powder.
In an apparent attempt to find a compromise, Klitschko travelled to the president's luxurious Mezhygirya residence outside Kiev to meet Yanukovych in person.
The president received Klitschko and promised early Monday to create a special commission of officials and opposition set up by national security council secretary Andriy Klyuyev to solve the crisis, the boxer's party and the presidency announced.
Klitschko told online television channel Hromadske TV that the president had appeared "very concerned" by the latest events but also pointedly ignored the opposition's main demand for early elections.
The boxer turned politician had earlier called on Yakukovych to "find it in yourself not to repeat the fate of (Nicolae) Ceausescu and (Moamer) Kadhafi", referring to the slain Romanian and Libyan dictators.
In the afternoon, some 200,000 people had filled Kiev's Independence Square and surrounding streets for a new mass rally in defiance of new strict curbs on protests passed by lawmakers in a show of hands last week and signed into law by Yanukovych.
The new laws allow the authorities to jail those who blockade public buildings for up to five years and permit the arrest of protesters who wear masks or helmets. Other provisions ban the dissemination of "slander" on the Internet.
Many of the demonstrators at the largely peaceful mass rally wore cooking pots and colanders on their heads while others sported ski, medical and carnival masks to mock the new legislation.
Protesters expressed frustration at the rally over the lack of a clear programme from the opposition leaders after almost two months of protests over Yanukovych's decision to ditch a pact with the EU under Russian pressure.
The opposition leaders were whistled and heckled during the main rally for their perceived inability to mount a stronger challenge.
"Unfortunately, they have not answered a question about a main leader," protester Ruslan Koshevarov said at the rally. "There's disappointment, the politicians have not met our expectations."
Yanukovych's arch nemesis Yulia Tymoshenko remains in jail, while the protest leadership appears riven by rivalries ahead of presidential election next year.