Dozens of demonstrators as well as hundreds of supporters gathered outside the building where the congress was taking place in the Romanian capital, where some 4,000 party members will elect 16 deputy presidents and a deputy leader at the meeting.
Supporters watched the congress which was broadcast on a screen while protesters yelled "You thieves!" and called the Social Democrats "The red plague." Police detained three people for disturbing public order.
Chairman Liviu Dragnea initially planned to seek a vote of support at the congress, but withdrew that option as some party members claimed he arbitrarily changed party rules.
The left-wing party retains its traditional support, but has been criticized after appointing three prime ministers in one year, and also for a tax overhaul which saw social security taxes shifted from the employer to the employee.