Thousands of socialists from across Bulgaria flocked to the capital for the annual May Day celebrations.

Demonstrators were waving both Bulgarian and European flags as they were marching through Tsar Osvoboditel Blvd to Sofia's Borisova Garden.

They made their traditional calls for "dignified" salaries and working conditions.

Police presence has been beefed up in the capital, but officers have observed no violence so far, according to the television channel bTV.

Bulgaria's Energy Minister Dragomir Stoynev, the Bulgarian Socialist Party's leader Sergei Stanishev and Parliament Speaker Mihail Mikov were among those who attended the rally to greet demonstrators and make an address to the crowd.

Some local media suggest the ruling party is using May Day celebrations for a pre-elections rally ahead of the European vote.

May 1, however, has always been perceived in Bulgaria as a festive occasion for the socialist party.