Several thousand people are on the streets protesting. Police have fired tear gas at stone-throwing demonstrators.
Sounds of gunfire have also been heard from central Martyrs' Square.
In a televised address, Lebanese PM Hassan Diab said he would ask for early elections as a way out of the crisis.
"We can't exit the country's structural crisis without holding early parliamentary elections," he said. The issue will be discussed in cabinet on Monday.
Many Lebanese are furious at the failure to prevent the explosion at a warehouse storing over 2,000 tonnes of ammonium nitrate.
The material had been seized from a ship six years ago but never moved. The government has promised to find those responsible.
The blast at the port devastated parts of the city and has deepened distrust of what many had already seen as an inept and corrupt political class.
An anti-government protest movement erupted last October, fuelled by an economic crisis and a collapsing currency.