Tetyana Chornovil, who writes for the Ukrainska Pravda opposition website, was attacked overnight Tuesday outside the capital Kiev, police said in a statement, citing the journalist.
The prominent journalist, known for her critical reports about Yanukovych and top officials, was driving to Kiev when she noticed she was being followed by a car.
"The driver of the suspicious car began to push her to the side. When she stopped, several men who were following her broke the back window of her car, pulled her out and started beating her," police said in a statement.
"After that she was thrown into a ditch," police said, adding she was found next to her vehicle shortly after midnight.
President Yanukovych condemned the attack and ordered Interior Minister Vitali Zakharchenko to find those responsible.
Later in the day Zakharchenko said three suspects in the beating had been identified and two of them detained.
Police said its "most experienced" investigators were probing the attack on the 34-year-old, who has participated in weeks-long pro-EU protests against Yanukovych.
Chornovil herself said there were "at least two" assailants.
"I started running, they began pursuing me," she said in video comments posted on the Ukrainska Pravda website.
"They were hitting me on the head, they were not saying anything, they were just hitting," said the young woman, her face bruised and swollen.
The news site said, citing relatives, that Chornovil was hospitalised with a broken nose, a concussion and multiple bruises.
The attack on the journalist comes after a local pro-EU activist was stabbed in both thighs in the eastern city of Kharkiv on Tuesday evening.
The latest assault caused outrage among opposition leaders, who have been locked in a standoff with Yanukovych over his decision to scrap key political and free trade agreements with the European Union last month.
Several hundred protesters on Wednesday gathered outside the seat of the interior minister, calling for his resignation.
Some of the protesters held up pictures of Chornovil.
'Last drop of blood'
"Today they nearly killed Tanya Chornovil and this should be the last drop of blood, the last manifestation of cruelty towards our people which we have all allowed through our inaction," jailed former prime minister Yulia Tymoshenko said in a statement.
"Police and bandits together are roughing up those who do not suit the authorities -- that's what the dictatorial regimes in Africa and Latin America did," opposition lawmaker Andriy Shevchenko said on Twitter.
World boxing champion and opposition leader Vitali Klitschko posted on Twitter pictures of a bloodied Chornovil as well as of several journalists injured in recent clashes between protesters and police.
"The price of freedom of the press in Ukraine now thanks to the system put in place by our current government," said the message next to the pictures.
The opposition has called for the resignation of Zakharchenko over police brutality and corruption but Yanukovych has refused to sack him.
Yanukovych's decision to scrap the EU pact sparked the largest protests since the pro-democracy Orange Revolution in 2004 but the demonstrations have been losing their momentum following a bailout deal with Russia last week.
Protesters have been occupying Kiev's central Independence Square known locally as Maidan since late November but opposition leaders have been unable to shake Yanukovych from his perch.
In an apparent bid to re-energise the protest movement, dubbed the "EuroMaidan", Tymoshenko on Wednesday encouraged protesters to march on her arch enemy Yanukovych's luxurious Mezhygirya residence on the banks of the Dnipro River.
She also urged the opposition to establish an interim government and a new central election commission.
The US embassy in Kiev said it was "appalled" by Chornovil's beating.
"We express our concern at 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 EuroMaidan protests," it said.
The EU Delegation to Ukraine noted "the increasing pressure on civil society activists, political leaders and journalists," while the Organisation for Security and Cooperation in Europe called on the authorities to improve the safety of journalists.
Source: The Sydney Morning Herald