By Duncan Kennedy
BBC News, Mexico City
Rivals in Mexico's Congress have exchanged punches, just days before President-elect Felipe Calderon is due to be sworn in to office.
PRD members say they plan to disrupt Friday's inauguration
The scuffle occurred after left-wing deputies tried to take the podium to protest against Friday's inauguration.
Deputies could be seen throwing punches and grabbing each other's clothes, and at least one man ended up on the floor.
Members of the left-wing party, the PRD, say Mr Calderon won July's presidential election by fraud.
The speaker of the chamber, Jorge Zermeno, suspended the session.
Members of the Party of the Democratic Revolution (PRD) have been saying for some time they intend to disrupt the inauguration ceremony of Felipe Calderon, to be held on Friday - and this may have been a dress rehearsal.
They claim Mr Calderon won the presidential election by fraud but Mr Calderon says he is determined to be sworn in at the Congress building.
Mr Lopez Obrador says fraud robbed him of election victory
Among the current and former leaders attending the event will be former US President George Bush. Tuesday's outburst may prompt a further review of the arrangements.
Last September, PRD deputies successfully blocked President Vicente Fox from giving his state of the nation speech in Congress by storming the stage.
Two weeks ago, PRD leader Andres Manuel Lopez Obrador declared himself the country's alternative president in another sign of protest against Mr Calderon.
It seems deputies from Mr Lopez Obrador's party are determined to support him wherever and however they can.