The search for those missing intensified, as rescuers sifted rubble in a race to find anyone still alive after Tuesday's explosion that killed 154 and smashed up a swathe of the city.

"The cause has not been determined yet. There is a possibility of external interference through a rocket or bomb or other act," President Michel Aoun said in comments carried by local media and confirmed by his office on Friday.

He said it would also consider whether it was a result of negligence or an accident. He previously blamed negligence in the storage of highly explosive ammonium nitrate for years at the port.

The United States has previously said it has not ruled out an attack. Israel, which has fought several wars with Lebanon, has denied it had any role.

Security forces fired teargas at a furious crowd late on Thursday, as anger boiled over at the government and a political elite, who have presided over a nation that was facing economic collapse even before the deadly port blast injured 5000 people.

The small crowd, some hurling stones, marked a return to the kind of protests that had become a feature of life in Beirut, as Lebanese watched their savings evaporate and currency disintegrate, while government decision-making floundered.

"Do we actually have a government here?" said taxi driver Nassim Abiaad, 66, whose cab was crushed by falling building wreckage just as he was about to get into the vehicle.

"There is no way to make money anymore," he said.

The government has promised a full investigation and put several port employees under house arrest. State news agency NNA said 16 people were taken into custody.

But for many Lebanese, the explosion was symptomatic of the years of neglect by the authorities while state corruption thrived.

Officials have said the blast might have caused losses amounting to $US15 billion ($A20.8 billion) - a bill the country cannot pay when it has already defaulted on its mountain of national debt, exceeding 150 per cent of economic output, and talks about a lifeline from the International Monetary Fund have stalled.

Hospitals, many heavily damaged as shockwaves ripped out windows and pulled down ceilings, have been overwhelmed by the number of casualties.

In the port area, rescue teams set up arc lights to work through the night in a dash to find those still missing, as families waited tensely, slowly losing hope of ever seeing loved ones again.

Lebanese Red Cross Secretary General George Kettaneh said three more bodies had been found in the search on Friday, while the health minister said the death toll had climbed to 154. Dozens are still unaccounted for.

Offers of immediate medical and food aid have poured in from Arab states, Western nations and beyond.

French President Emmanuel Macron came to the city on Thursday with a cargo from France. He promised to explain some "home truths" to the government, telling them they needed to root out corruption and deliver economic reforms.

He was greeted on the street by many Lebanese who asked for help in ensuring "regime" change, so a new set of politicians could rebuild Beirut and set the nation on a new course.

Australian Associated Press