KIEV - Protesters demanded Ukraine's interior minister resign on Thursday after an opposition journalist known for documenting the extravagance of the country's political elite was chased down in her car and savagely beaten in a midnight attack.

Clutching pictures of Tetyana Chornovil's badly bruised face, hundreds marched on the Interior Ministry in the capital, Kiev.

The attack on the 34-year-old restored passion to protests that have been losing steam more than a month after the government spurned a pact on closer ties with the European Union, turning instead to Moscow.

Pro-EU demonstrators have been occupying central Kiev, but their numbers have been falling since Russia offered Ukraine a $15 billion bailout this month.

The United States said the journalist's beating was "particularly disturbing" and that Washington was concerned and watching the situation.

"The United States expresses its grave concern over an emerging pattern of targeted violence and intimidation towards activists and journalists" who participated in or reported on the protests, said a statement issued by State Department spokeswoman, Jen Psaki on Thursday evening.

Psaki called on Ukraine's government to ensure respect for human rights and to send an "unequivocal message" that violence against government critics "will not be tolerated."

"The United States, in concert with our European partners, will continue to closely watch the disposition of this and other cases," she said in the statement.

Ukraine's interior minister, Vitaly Zakharchenko, had already become a target of opposition anger following a violent crackdown on protesters by police late last month that helped swell the demonstrations.

The attack on Chornovil, shortly after midnight on Wednesday, came hours after she posted pictures online of what she said was Zakharchenko's home, part of a campaign to expose the opulence of the political elite under President Viktor Yanukovich.

"Our police no longer protect their people, but fight them instead, hurt and oppress them," said protester Valentina Gorilova, a 47-year-old housewife.

Some protesters, their hands chained, knelt before a row of police in mock supplication.

The opposition movement has shown signs of waning. A hard core of hundreds continue to camp out around braziers on Kiev's Independence Square, swelled by weekly mass rallies of around 100,000 or more.

BILLIONS IN RUSSIAN AID

Chornovil, who has played an active role in the protests, shot to prominence last year when she infiltrated the grounds of Yanukovich's opulent residence in a park near the Dnieper River.

She has since posted photographs online of the homes of other senior officials. Zakharchenko was her target on Tuesday. "Here lives the executioner," the journalist wrote in her blog, above pictures of a handsome country property.

Hours later, Chornovil was chased on a road outside the capital, a dashboard camera capturing how a black Porsche Cayenne veered and rammed into her car before at least two men jumped out.

"When a very, very posh car is ramming you first on the side, then from behind, then from the front you understand that they've been paid already for your life," a beaten and bloodied Chornovil told local television station Channel 5, laying on a hospital bed.

Her lips were swollen and split, one eye blackened and closed by bruising.

"I am a Revolution activist ... and I am very well known as a journalist, I've made a number of journalistic investigations which ... are irritating them (government officials)," she said.

The Organisation for Security and Cooperation in Europe (OSCE) and the United States embassy in Kiev condemned the attack.

The embassy noted "a strikingly similar series of events over the last few weeks, targeting individuals, property, and political activity, apparently aimed at intimidating or punishing those linked to the ... protests."

"We condemn the attack and call for an immediate investigation, which unlike previous such incidents must result in those responsible being held fully accountable under the law," it said in a statement.

Media reports said another opposition activist was stabbed in the eastern city of Kharkiv on Tuesday.

Yanukovich on Wednesday called on police to find those responsible for the attack on Chornovil. Two men were detained later on the same day and police said they had detained a third suspect on Thursday.

The president's pivot away from Europe last month has thrown the country of 46 million people into turmoil, exposing a deep rift among Ukrainians over whether their future lies with the EU or Russia.

Rejecting the EU trade deal, Yanukovich turned instead to Russia for an aid package worth $15 billion to help ease a worsening financial crisis. It received a first $3 billion tranche this week.

On Thursday, ratings agency Standard and Poor's revised the outlook on Ukraine's long-term sovereign 'B-' rating to stable from negative, saying the bailout would cover the country's financing needs over the next year.