The police chief in the Ukrainian port city of Odessa, where unrest left more than 50 people dead, has been sacked, Prime Minister Arseniy Yatsenyuk has said.
Yatsenyuk made the announcement of Pyotr Lutsyuk's dismissal on Sunday hours hours after the country's interim president declared two days of mourning.
He said an investigation into the unrest would be carried out.
In a statement announcing national mourning, interim president Oleksandr Turchynov said: "The day of May 2 was a tragic day for Ukraine. I have signed a decree for two days of mourning in Ukraine for the heroes who died in the course of the anti-terrorist operation and also for those who died in the tragic events in Odessa."
Al Jazeera's Jonah Hull, reporting from Odessa, said that a large crowd had gathered outside a police station in the city.
"It appears as though they are demanding the release of a number of people arrested on Friday. It is an extremely angry crowd.
"There is a contingent of police outside, and the crowd are chanting fascists, fascists, fascists."
At least 42 people lost their lives in Odessa on Friday after fighting between pro-Russian and pro-Ukraine armed groups climaxed in an inferno that trapped dozens as both sides hurled petrol bombs.
Nine more people, including four servicemen, died in fighting on the same day as the Ukrainian army intensified what the government calls an "anti-terrorist" operation around the rebel-held town of Slovyansk.
The Kremlin and Ukraine traded accusations over the violence, with Kiev saying the Odessa violence had been "coordinated by sabotage groups from Russia".
Moscow said it was "outraged" as the scenic port city became a new front in an escalating months-long crisis that has sparked fears of a Russian invasion.
Our correspondent in Odessa said the people there did not accept Kiev's version of events, and that the mourning period and firing of Lutsyuk would not reduce the tension in the city.
"People walking around the building that was on fire on Friday are dazed and shocked by what happened," he said. "They reject the idea of Russian involvement and place the blame on the government in Kiev."
Meanwhile, European military observers held for more than a week by pro-Russian separatists arrived in Germany a day after they were released in Slovyansk.
The seven observers from the Organisation for Security and Cooperation in Europe (OSCE) - along with five of their Ukrainian assistants - were freed on Saturday.
They were deployed following a pact struck between Russia, Ukraine, the European Union and the US in mid-April aimed to resolve the crisis, which began with Russia's annexation of Crimea.
"We are deeply relieved that the members of the kidnapped OSCE team have landed unharmed here in Germany, in Berlin," Ursula von der Leyen, Germany's defence minister, said in Berlin.
"I would like to express my deep gratitude and my respect for the infinitely good cooperation we saw."
Nicolai Wammen, Denmark's defence minister, said they would continue to support the OSCE mission.
"It is important that they can work and conduct their important business in Ukraine and that this event will not make that more difficult in the future," he said.
The mission's prospects became clouded a week after their deployment when they were detained by armed men in Slovyansk, the crucible of unrest in eastern Ukraine.