The move came after heavy fighting with pro-Russia militants overnight, amid reports of casualties.
Pro-Russia separatists reentered the town hall after Ukrainian forces apparently left the building, raising the Russian and separatists' flags again.
Earlier, separatists told the Interfax news agency that Ukrainian security forces detained several people in Mariupol who were campaigning for a referendum on the region's independence scheduled for May 11.
Meanwhile, today Ukrainian forces reportedly detained Ihor Kakidzyanovk, the defense minister of the self-proclaimed Donetsk People's Republic today after a shoot-out near the town of Mangush, some 20 kilometers east of Mariupol.
There were also reports of fighting around Slovyansk, where pro-Russia insurgents attacked government forces surrounding the rebel-held city.
Polish Foreign Minister Radoslaw Sikorski said today that President Bronislaw Komorowski will hand the prize to Dzhemilev at a special ceremony in Warsaw in June.
Sikorski said that Dzhemilev has been fighting for human rights since Soviet times, and spent almost 10 years in Soviet prisons for his efforts to protect the rights of his people.
Dzhemilev is known for his support of Ukraine's territorial integrity.
The annual Solidarity Prize was established in January as part of the 25th anniversary celebrations of Poland's historic June 4, 1989 elections, which led to the first democratically elected government in Poland after World War II.
#Ukraine govt says it controls municipal building in #Mariupol. But it doesn't. We're there - pro #Russians got it back.
— Lindsey Hilsum (@lindseyhilsum) May 7, 2014
#mariupol city council building back in pro Russian hands pic.twitter.com/6bmHw5tAkd
— Simon Denyer (@simondenyer) May 7, 2014
Burning #Ukraine flag in front of municipal building in Mariupol, now back in pro #Russian hands. #c4news pic.twitter.com/pkYDooYTiq
— Lindsey Hilsum (@lindseyhilsum) May 7, 2014
Catherine #Ashton: #Ukraine has the right to defend its territorial integrity + have done a lot to implement what was agreed in Geneva.
— EU Delegation UA (@EUDelegationUA) May 7, 2014
Mariupol city council building pic.twitter.com/Lp5HTHT9r6
— Simon Denyer (@simondenyer) May 7, 2014
Jeffrey Feltman, the UN undersecretary-general for political affairs held talks today with Ukraine's acting president Oleksandr Turchynov.
British Foreign Secretary William Hague is also in Ukraine for talks with the nation's leaders.
The U.S. and European nations have increased diplomatic efforts ahead of Ukraine's May 25 presidential election.
Meanwhile, Ukrainian forces today pushed into the eastern port of Mariupol, occupying the town hall shortly after heavy fighting overnight. Casualties were reported.
The town hall was retaken by pro-Russia separatists after Ukrainian forces left the building.
Ukrainian troops reportedly detained Ihor Kakidzyanovk, the defense minister of the self-proclaimed pro-Russian Donetsk People's Republic, near Mariupol today.
Fighting was also reported around the flashpoint city of Slovyansk.
Ukraine police defend police station where separatists detained. Crowd outside very angry pic.twitter.com/PLnT3Mw7g1
— Simon Denyer (@simondenyer) May 7, 2014
Traffic in central #Sloviansk, #Ukraine. Save for the rumbling of an occasional APC, city is quiet. Very quiet. pic.twitter.com/2TxhMEwqc0
— Christopher Miller (@ChristopherJM) May 7, 2014