"We, members of the army and revolutionaries (former rebels), announce the suspension of the General National Congress," said Mokhtar Fernana, reading out a statement broadcast.
Earlier, heavy gunfire and clashes broke out in the south of the Libyan capital, near the country's interim parliament whose members were evacuated.
Witnesses and an MP said the clashes erupted on Sunday after a convoy of armoured vehicles entered Tripoli from the airport road and headed for the General National Congress (GNC).
GNC members were evacuated after gunmen in civilian clothes attacked the building, according to a member of the interim parliament, who was unable to identify the assailants.
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But some witnesses said the gunmen were members of the powerful Zintan brigades, made up of former rebels who fought late dictator Moamer Kadhafi in 2011.
The groups from Zintan now control areas in southern Tripoli around the airport.
They said a column of smoke billowed over the GNC building after gunmen set fire to an annex, and that several cars parked nearby had been damaged.
Roads leading to parliament were also sealed off, as Tripoli residents rushed home.
Later, the gunmen were seen pulling back to their base and gunfire was heard along the airport road, witnesses said.
Militias have launched several attacks on the GNC, including on March 2, when two MPs were shot and wounded.
The latest violence in Tripoli came after deadly fighting in the eastern city of Benghazi on Friday, when a former general unleashed his so-called National Army on Islamist militiamen.
It was not immediately clear if there was any link between the unrest in Tripoli and the Benghazi clashes.
At least 79 people were reported killed in the eastern city of Benghazi.
The Zintan brigades are known for their staunch opposition to Islamists.
