There were no immediate reports of casualties in Tuesday's attack on the offices of the Iraqi National Accord, according to Hassan Attan, assistant to the city's governor.
"The building's guards responded and security forces cordoned off the area and went in search of the attackers," he added.
On Sunday, Allawi was attacked with shoes and tomatoes thrown by a mob when he attempted to pray at the Imam Ali mosque in Najaf. He accused his assailants of attempting to kill him.
"What happened is all part of the amoral campaign targeting the party and its members," said Abdel Aal al-Issawi, a party official in Najaf, at the time.
Interior Minister Bayan Baker Solagh announced a committee had been formed to investigate the matter.
Allawi, who was the country's first prime minister following the handover of power from the US-appointed Coalition Provisional Authority, is looking to stage a comeback in the 15 December elections, after his party's disappointing showing in January's poll.