Mr de Rossa is a well-known figure in Irish politics
Irish former minister Proinsias de Rossa was knocked to the ground after a public meeting in Dublin on Monday night, according to the Labour Party.
Labour leader Eamon Gilmore said the MEP was confronted by a group of men who screamed abuse at him before knocking him over and pinning him down.
Police confirmed the attack which left Mr de Rossa bruised and shaken.
The Irish Republic is the only EU member state to have a public vote on the Lisbon Treaty, on 12 June.
Labour is campaigning for a Yes vote in the referendum and has called for a calm and rational debate about the treaty.
EU leaders signed the document in Lisbon last December with the aim of making EU institutions more efficient, now that the bloc has 27 member states.
In his statement Mr Gilmore said a minority of people had tried to disrupt the meeting, using "ugly and intimidatory" tactics and shouting abuse through a loud-hailer.
"I was greatly shocked by what happened to Proinsias de Rossa. In a democracy elected representatives must be free to walk the streets without the threat of physical abuse," he said.
Police took statements from witnesses but made no arrests.
Mr de Rossa is a well-known figure in the Irish Republic, having served as a cabinet minister in the 1990s and as an MEP since 1999.
He was unavailable for comment.