The militants also had banners with neo-Fascist symbols, ANSA reported.
Journalists and cameramen were threatened, the Italian news agency said.
Rome Mayor Gianni Alemanno and former Senate caucus leader Maurizio Gasparri were among 1,000 people to attend the funeral, ANSA said.
Both men are members of former Prime Minister Silvio Berlusconi's People of Freedom Party and are former members of the Italian Social Movement, the neo-Fascist party created at the end of World War II.
Alemanno gave Buontempo, who was known for his humor, the honor of lying in state at the Capitol, ANSA reported.