The elections coincided with a regular visit to Lisbon by inspectors from the so-called 'troika' of its bailout creditors the country's fellow euro members, the European Central Bank and the International Monetary Fund who reportedly are reluctant to ease austerity measures.
The political uncertainty in Portugal could help undermine efforts by the 17 countries that share the euro currency to draw a line under their three-year-old financial crisis.
The municipal elections do not directly affect the government, but austerity measures were at the heart of the main parties' election campaigns.
Social Democrat leader and Prime Minister Pedro Passos Coelho conceded his party suffered a heavy defeat but said late Sunday he can't halt his unpopular program of tax hikes, pay and pension cuts and reductions in public services because the bailout creditors would stop handing over the rescue funds.
An expected third straight year of recession in 2013 and a jobless rate of 16.5 percent have turned many people against the bailout terms, which initially had broad support, including from the Socialist Party.