Ukrainian President Viktor Yanukovych has sacked the head of the armed forces, Col Gen Volodymyr Zamana, according to the president's website.
The move comes after the most intense violence in Ukraine's three-month crisis turned Kiev into a battle zone.
Earlier the state security service announced it was launching a nationwide "anti-terrorist" campaign to deal with a growing "extremist threat".
There was a suggestion the armed forces could be deployed for the first time.
Col Gen Zamana has been replaced by the commander of Ukraine's navy, Admiral Yuriy Ilyin, by presidential decree, President Yanukovych's website said.
The news comes after the most deadly violence in Ukraine's post-Soviet history erupted on Tuesday and overnight.
At least 26 people died, mainly in Kiev, as protesters wielding petrol bombs and paving stones tried to defend their encampment in the central Independence Square from police using rubber bullets and stun grenades.
The crisis was triggered when President Yanukovych pulled out of an association deal with the EU.
Since late November, that decision has fostered a militant protest movement intent on seeing him ousted from power.
It is so far unclear what prompted Mr Yanukovych's decision to dismiss the commander-in-chief of the armed forces - who has been in place two years and a day, according to his official biography.
But it came hours after the head of the top security agency, Oleksandr Yakimenko, said an "anti-terrorist operation" was being launched after "extremist groups" seized government buildings and arms depots.
Mr Yakimenko did not provide details of what the anti-terrorist operation would involve, but he did say that - according to Ukrainian law - the country's interior ministry, border guards and armed forces could become involved.
Ukraine's defence ministry has also said it is redeploying units around the country to guard military facilities.
Several European leaders have condemned the Ukrainian leadership for the violence.
The foreign ministers of France, Germany and Poland are to meet in Kiev on Thursday to assess the situation before an EU meeting in Brussels to decide whether to impose sanctions against Ukraine.
US President Barack Obama warned there "will be consequences" for anyone who steps over the line in Ukraine - including the military intervening in a situation that civilians should resolve.
UN Secretary General Ban Ki-Moon said in a statement the use of violence in Ukraine "by any side is totally unacceptable" and urged the authorities to "desist from the use of excessive force".
Russia, meanwhile, has characterised the violence as an "attempted coup" by extremists it says are egged on by Western countries.
The protests began in late November, when President Yanukovych rejected a landmark association and trade deal with the EU in favour of closer ties with Russia.
Tensions had begun to subside as recently as Monday, when protesters ended their occupation of government buildings in Kiev in return for an amnesty from prosecution.
But violence erupted outside parliament on Tuesday morning as government supporters blocked opposition attempts to scale back the president's constitutional powers. They argued more time was needed to discuss the proposals.
Police have been trying to wrest control of Kiev's Independence Square, also known as the Maidan, which has been in the hands of protesters for several months.
In apocalyptic scenes overnight on Tuesday, protesters hurled paving stones, fireworks and petrol bombs while police used water cannon and stun grenades. Both sides accused the other of using live ammunition.
On Wednesday, the violence subsided but there were still periodic clashes and protesters are reported to have seized the central post office.
The health minister says at least 600 people have been wounded, more than half of them police officers.
Unrest has also been reported outside Kiev, with a woman reportedly shot dead during an attempt to storm a building belonging to the security services in Khmelnytskyy.
Protesters seized regional administrative and police buildings in the western city of Lviv, and occupations and attacks on police buildings were also reported in Ivano-Frankivsk, Ternopil and elsewhere.
The number of dead on both sides has climbed to 26 and it is feared the death toll could increase. As well as the woman reportedly shot in Khmelnytskyy, those killed include:
- Ten police officers, according to the interior ministry, two of them traffic officers
- At least 14 protesters, many killed in the streets around the parliament
- A journalist working for Russian-language newspaper Vesti, Vyacheslav Veremyi, who was pulled from a taxi by masked men and shot dead
'Crossed the line'
Opposition leaders Vitaly Klitschko and Arseniy Yatsenyuk met President Yanukovych for late-night talks on Tuesday but failed to come to an agreement.
In a statement broadcast on TV news channels on Wednesday morning, President Yanukovych said: "The opposition leaders have disregarded the principle of democracy according to which one obtains power not on the streets or maidans - but through elections.
"They have crossed the line by calling for people to take up arms."
"This is an island of freedom and we will defend it," said Mr Klitschko, the former boxer and leader of the Udar (Punch) party.
Mr Yatsenyuk, who heads the Fatherland party, appealed to President Yanukovych to "stop the bloodshed and call a truce".
The state of Ukraine's stagnating economy is a major factor in the crisis. After 18 months in recession, the country is facing a rising trade and budget deficit, and mounting foreign debt.
The European Investment Bank (EIB), the EU's soft loan branch, said it had frozen its activities in Ukraine on Wednesday. Since 2007, it has invested more the $2bn (£1.2bn) in Ukraine in projects including the extension of a metro line, and modernisation of air-traffic control facilities.
Earlier this week, Russia said it would resume aid payments to Ukraine as part of the a $15bn (£9bn) loan plan agreed in December. But the Russian president's spokesman, Dmitry Peskovhas, has since said that the next Russian bailout payment was on hold "because the priority is to settle the crisis".