The 63-year-old president appears increasingly isolated in a crisis born of a tug-of-war between the West and Russia. A former president said this week that the violence between demonstrators and police had brought the country to the brink of civil war.
Shortly after his office announced he had developed a high temperature and acute respiratory ailment, Yanukovich defended his record in handling the crisis and accused the opposition, which is demanding his resignation, of provoking the unrest.
"We have fulfilled all the obligations which the authorities took on themselves," a presidential statement said, referring to a bill passed late on Wednesday granting a conditional amnesty for activists who had been detained.
"However, the opposition continues to whip up the situation, calling on people to stand in the cold for the sake of the political ambitions of a few leaders."
The amnesty offered freedom from prosecution to peaceful protesters, but only on condition that activists left official buildings they have occupied - something they have rejected.
Several members of Yanukovich's own party voted against the bill, even after he visited parliament himself to rally support, and some of his powerful industrialist backers are showing signs of impatience with the two-month-old crisis.
"The president of Ukraine has been officially registered as sick, with an acute respiratory ailment and a high temperature," a statement on the presidential website said.
A subsequent statement gave fullsome tribute to a police officer who was found dead early on Thursday, apparently from a heart attack while on duty - an indication of how important Yanukovich regards keeping the security forces on his side.
The bare announcement on his health gave no sign of when he might be back at his desk or able to appoint a new government, which Moscow says must be in place before it goes ahead with a planned purchase of $2 billion of Ukrainian government bonds.
"Today is the first day of the illness. He has a high temperature. We are not doctors, but it is clear that a high temperature does not go down in a single day," a presidential spokesman said by telephone. "The doctors will do all they can so that he can recover quickly."
Some opposition figures said they suspected Yanukovich might be giving himself a breathing space after being forced into concessions to try to calm the unrest on the streets.
"This smacks of a 'diplomatic illness'," Rostislav Pavlenko, a member of boxer-turned-politician Vitaly Klitschko's Udar (Punch) party, told Reuters. "It allows Yanukovich not to sign laws, not to meet the opposition, absent himself from decisions to solve the political crisis."
A close ally of Yanukovich, who was last seen in parliament on Wednesday night, rejected that interpretation.
The president had hurried to the legislature to herd supporters into voting for the amnesty bill. Mykhailo Chechetov, from Yanukovich's Party of Regions, said the president had told supporters there he had come to the session directly from hospital. "He looked ill," Chechetov said.
Photographs released by the presidential press service of Yanukovich holding talks with a European Union delegation earlier in the day revealed no obvious signs of illness.
In a statement the three main opposition leaders, including Klitschko, accused Yanukovich of ignoring violations of voting procedure in the Wednesday night vote.
"Viktor Yanukovich bears responsibility for the violations of constitutional norms ... [he] personally went to parliament and by blackmail and intimidation forced his faction, which is balanced on the edge of a split, to go back in and push through a law even when there were not enough votes for it," they said.
Thirty-year-old Ruslan Andriyko, one of the hundreds of protesters occupying Kyiv's City Hall, said it would not work.
"We will clear this building only if we get the resignation of Yanukovich, which is the main aim of our revolution, and the approval of the people on the 'Maidan' [Kiev's Independence Square]," he said.
Former Polish President Aleksander Kwaśniewski said he believed the hasty visit to parliament was a sign Yanukovich, who he has met many times, was afraid of losing support.
"I think this urgent visit by the President to parliament shows he is afraid that the majority is no longer on his side," Kwaśniewski said on Polish radio.
Oligarchs stay put
Ukraine's richest entrepreneurs, whose support Yanukovich has had and needs now, are now taking a more neutral line.
Chemical and gas billionaire Dmitry Firtash called on all sides in the conflict to find a compromise by negotiations that would yield "real" results, according to a statement from him on Thursday. Ukraine's richest man, steel magnate Rinat Akhmetov, made a similar appeal earlier this week.
Yanukovich's most urgent task now is to appoint a successor to Azarov, who served him loyally for four years, while the opposition is anxious that he also signs into force a repeal of anti-protest legislation.
Ukraine badly needs a new government. Russian President Vladimir Putin said on Wednesday Moscow would wait until one was formed before fully implementing the $15 billion (€11 billion) bailout deal.
Kerry meets opposition leaders
U.S. Secretary of State John Kerry will meet key Ukrainian opposition figures on Friday on the sidelines of a security conference in Munich, a senior U.S. official said on Thursday, his first meeting with some of the leaders of an anti-government uprising against President Viktor Yanukovich.
Washington has welcomed talks in recent days between Yanukovich and the opposition groups to end two months of protests that began when Yanukovich rejected an EU trade deal in favor of closer ties and a financial bailout with Russia.
Among those attending the meeting with Kerry is Yatsenyuk, Klitschko, Petro Poroshenko, a member of parliament; and Ukrainian pop star Ruslana Lyzhychko.
Discussions with Kerry would focus on ways to restore calm and relaunch the political process in Ukraine, including plans to form a new government, according to the official.
Abducted activist tortured
In the meantime, Ukrainian media report that a prominent Ukrainian antigovernment activist, who went missing more than a week ago, has turned up in a village near Kyiv, saying that he was kidnapped and tortured by unknown men who spoke with Russian accents.
Dmytro Bulatov, the 35-year-old spokesman for the Automaidan protest group, was reported missing on 23 January -- just after the abductions of two other opposition activists, one of whom was found dead in a forest near Kyiv with signs of being tortured. RFE/RL’s Ukrainian Service reports that Bulatov was taken to a Kyiv hospital after turning up on the night of 30 January. He was being treated for injuries from apparent beatings and torture -- including a severed ear and puncture wounds in his arms.
Bulatov was quoted by opposition figures as saying that his abductors had hung him up as if he were being crucified before eventually taking him down, throwing him into a car, and dumping him in the countryside. Bulatov was also quoted as saying that after he was abandoned in freezing temperatures on the night of January 30, he managed to walk to a nearby village where he telephoned friends who took him back to Kyiv.