Pro Democracy chairman Cristian Parvulescu said his observers found many irregularities, but the group could not determine whether there had been large-scale fraud.
"The law, offering Romanians the possibility to vote in any polling station around the country, created the possibility of election fraud," Parvulescu said.
Pro Democracy said later it would refuse to monitor the presidential runoff to protest irregularities.
The only other parties that cleared the 5 percent threshold to enter Parliament are the far-right nationalist Greater Romania Party, which scored about 13 percent, and its rival ethnic Hungarian Party, which netted just under 7 percent.
The Greater Romania Party is led by flamboyant poet-turned-politician Corneliu Vadim Tudor, known for his virulent rhetoric against Jews, Hungarians, Gypsies and other minorities.
Tudor recently apologized for past anti-Semitic statements and has toned down his nationalist message, but analysts say he and his party are still unacceptable to Western governments.
Twelve candidates ran for president, and Tudor came in third with about 13 percent. But both leading parties, the Social Democrats and the Alliance, have ruled out making any deals with the nationalists.
About 60 percent of Romania's 18 million eligible voters turned out, officials said.