They turned out to voice their opposition to austerity measures and planned health care reforms.
But a protest in the Romanian capital Bucharest turned violent when protesters refused to leave. They scuffled with riot police, who used tear gas.
Authorities said nine people were injured in the clashes. Nearly 30 arrests were made.
It was the fourth day of demonstrations in Romania against cuts and falling living standards. The protests are the most serious since President Basescu come to power in 2004.
As its economy shrank in 2009, Romania took out a 20 billion euro loan from the IMF, EU and World Bank. It has had to bring in harsh austerity measures as a result.