Thousands of people gathered in Bucharest to celebrate after an ethnic German Transylvanian mayor decisively defeated Romania's prime minister in a presidential runoff.

Prime Minister Victor Ponta had been the favorite to win. But Klaus Iohannis, the mayor of the city of Sibiu who promised to crackdown on corruption scored a surprise victory.

With about 76 percent of the votes counted, Iohannis was leading with about 54.8 percent and Ponta 45.2 percent early Monday.

Late Sunday, Ponta conceded defeat.

"Thanks to you, another kind of Romania is beginning," Iohannis said on his Facebook page.

Some 15,000 had gathered to protest difficulties encountered by expatriate Romanians voting abroad. But the protest turned into a celebration as Iohannis walked through the jubilant crowd gathered in a square where demonstrators were shot dead during Romania's 1989 revolution.