Witnesses said supporters of President Hosni Mubarak's ruling party beat and kicked activists who were urging a boycott of the 25 May referendum and journalists who covered the protest.
Some women journalists and protesters said they were molested by government supporters.
Opposition al-Ghad (Tomorrow) Party leader Ayman Nour called for a parliamentary committee to investigate the assaults, but a majority of some 100 MPs rejected the request at Saturday's session. Mubarak's party dominates parliament.
"We just want the truth," Nour told parliament. The public prosecutor has already launched an investigation into the violence during the protest by the Kifaya (Enough) Movement, which is opposed to an extension of Mubarak's 24-year rule.
Egypt's presidential spokesman last week said the media had overplayed the attacks, but such incidents were still unacceptable.
The new election system, approved in the referendum, opens the way for Egypt's first multi-candidate presidential poll.
But the opposition says it will guarantee victory for the incumbents by setting tough conditions on who can run and perpetuate the rule of Mubarak's National Democratic Party.