Pro-Russian separatists have seized back the city hall in the southern Ukrainian port of Mariupol, hours after being ousted by security forces.
Government forces first raised the Ukraine flag on the building but later left - allowing the rebels who captured it last week to be back in control.
Kiev has sent in troops to try to take back official buildings occupied by pro-Russian rebels in recent weeks.
Meanwhile, Russia has urged rebels to postpone an independence referendum.
President Vladimir Putin said the series of votes, planned for Sunday in south-eastern Ukraine, should be postponed "in order to create the conditions necessary for dialogue".
He also said, after talks with the chair of the Organization for Security and Co-operation in Europe (OSCE) in Moscow, that he was ready to "seek ways out of this crisis".
Russia was again accused, by UK Foreign Secretary William Hague, who was meeting Ukrainian leaders in Kiev, of "trying to orchestrate conflict and provocations" in eastern and southern Ukraine.
Mr Hague said Moscow's "immediate goal" was to disrupt Ukraine's presidential election on 25 May, "although of course they might also be trying to provide a pretext for intervention".
Earlier on Wednesday, Ukrainian Interior Minister Arsen Avakov said government troops had taken over Mariupol's city hall following a joint operation by ministry troops and the army.
Mr Avakov told the BBC the defence minister of the separatist "Donetsk People's Republic", Igor Kakidzyanov, was among a number of people detained.
Shortly after the Russian and Donetsk Peoples' Republic" flags were hoisted over the city hall, the scene of tensions moved to the police station where the detainees were being held.
Heavily-armed police fired warning shots into the air as an angry crowd of friends and relatives gathered at the police gates concerned that the detainees would be moved to another province, the BBC's Sarah Rainsford reports from the scene.
There were also reports of clashes in the separatist stronghold of Sloviansk, as Ukrainian troops advanced on rebel positions.
The Kiev government says 14 Ukrainian soldiers have been killed and 66 wounded since the start of its operation in the east last month.
In related developments on Wednesday:
- The government has asked Ukraine's football league to hold remaining matches in the season without spectators and for those in the Donetsk, Luhansk, Odessa and Kharkiv regions to be moved to other areas
- Ukraine's central bank has received the first $3.19bn (£1.9bn; 2.3bn euros) tranche of a loan previously agreed with the IMF - part of an overall package of $17bn.
The government has rejected the pro-Russian activists' demands for greater autonomy for eastern regions, fearing they could lead to the break-up of the country.
Kiev has said it is ready to back new international talks in Geneva, as long as Moscow supports its presidential poll.
On Tuesday, Russian Foreign Minister Sergei Lavrov ruled out holding another round of international talks unless pro-Russian opposition groups were involved.
He said there was no point attending as an accord agreed in April between the US, EU and Russia had not been implemented.
The Geneva agreement called for all parties, including the separatists, to refrain from violence, disarm and vacate occupied buildings.
Tensions have been high since Kremlin-backed forces seized control of the Crimean peninsula, which then voted to join Russia in a March referendum that Kiev and the West deemed to be illegal.