A statement posted on Viktor Yanukovich’s presidential website stated he “has been officially registered as sick with an acute respiratory ailment and a high temperature”.
But his office gave no indication of how long the illness was expected to keep him from the political fray and whether or not he would carry out his duties from his sick bed.
The lack of both information and a president creates a power void at a time when anti-government demonstrators still occupy chunks of central Kiev and fears remain high that the crisis could spiral out of control.
Mr Yanukovich went on sick leave hours after he had made a rare visit to parliament during a tense vote on an amnesty law that would free people arrested during the demonstrations.
A key demand of the anti- government forces, it was adopted but with significant conditions that are likely to be rejected. One demands demonstrators leave Maiden Square – the centre of the protests – within 15 days.
Vitali Klitschko, leader of the opposition Udar Party, said: “This law, like many others, does not meet the interests of people. And this voted law seems to be like the law of Somali pirates, who take you hostage and then let you free for a ransom. Instead of lowering the temperature in society, this is going to raise it.”
The dissatisfaction with the amnesty law provoked a stinging response from Mr Yanukovich, who fired a verbal broadside at the opposition before falling ill.
“The government has fulfilled all its obligations under these agreements including the adoption of the law on amnesty that guarantees freedom and liberation of persons arrested during the conflict,” he said. “Why do politicians not call for peace and mutual understanding but kindle the emotions with their reckless and irresponsible statements, thinking more about their ratings than about the life and health of people?
“We must understand there is no future for the state and people if political interests of certain groups are set higher than the existence of Ukraine itself.”
But, adopting a softer tone, he admitted the authorities had made mistakes. “From my side, I will show more understanding for the demands and ambitions of people, taking into account the mistakes that authorities always make,” he said.
The president’s retreat into the temporary political sanctuary of illness came as rumours of discord and discontent within his Party of Regions, the dominant force in Ukrainian politics, run rife.
Some have attributed his surprise appearance in parliament on Wednesday night to his need to quell internal rebellions and force people to tow the line.
Aleksander Kwasniewski, a former Polish president who has met the Ukrainian leader many times as part of a European Parliament monitoring mission to Ukraine, said the president was running out of friends. “I think this urgent visit to parliament shows he is afraid that the majority is no longer on his side,” Mr Kwasniewski said.
Since the start of the conflict, after the government’s decision to walk away from a key agreement with the EU, Ukraine has been awash with rumours that some of the country’s powerful oligarchs disagree with Mr Yanukovich’s position, and may have even persuaded him from declaring a state of emergency. Unlike in neighbouring Russia, where the oligarchs must remain loyal to Vladimir Putin, Ukraine’s rich and powerful enjoy a certain amount of freedom.