The takeover, aired live on Lebanese television, came as security forces had their attention focused on a tense demonstration against the ruling elite a few hundred metres (yards) down the road, as clashes added to the city's havoc.
"We are taking over the foreign ministry as a seat of the revolution," said Sami Rammah, a retired officer who spoke through a loudspeaker from the front steps of the ministry building.
"We call on all the anguished Lebanese people to take to the streets to demand the prosecution of all the corrupt," he said.
Near the site of the main demonstration, where nightfall came with familiar scenes of stone-throwing youth clashing with tear-gas firing security forces, a group of protesters briefly entered the economy ministry and seized piles of documents they scattered on the street.
Another group of demonstrators attempted a similar move against the headquarters of the Association of Banks in Lebanon that has in recent months been a focus of protesters' ire.
The explosion at Beirut port Tuesday that is widely blamed on the incompetence and corruption of the ruling elite killed at least 158 people, wounded 6,000 and made hundreds of thousands homeless.
Rammah, who stressed the protest camp was "not against one specific person but against a system that destroyed the country", urged the international community to boycott the government.
"We call all our Arab allies and other allied nations and the Arab League and the United Nations to consider our revolution as the real representative of the Lebanese people," he said.
His call came amid intense diplomatic activity in solidarity with disaster-struck Lebanon and on the eve of an international donor conference.
Four days since the blast, grief gave way to rage, with the large demonstrations reminiscent of the unprecedented nationwide and cross-sectarian protest movement that erupted in October.
Around 200 people occupied the foreign ministry compound, located on an upmarket street in central Beirut.
Their entry into the ministry appeared to have been facilitated by the damage the historical building sustained in Tuesday's monster explosion.
Some of the protesters ripped the portrait of President Michel Aoun off the wall and smashed it on the ground, an AFP correspondent said.
