El Salvador's President Nayib Bukele and a group of soldiers armed with automatic weapons have briefly occupied the country's Congress, stepping up a campaign pressuring MPs to back a crime-fighting plan.

Key points: Authorities in El Salvador continue to battle gangs for control of vast territory

Authorities in El Salvador continue to battle gangs for control of vast territory Mr Bukele is seeking backing for a crime-fighting plan

Mr Bukele is seeking backing for a crime-fighting plan Human Rights Watch have called Mr Bukele's actions an "exhibition of brute force"

Watched by soldiers in full battle uniform, Mr Bukele, 38, sat in the seat reserved for the president of Congress and cupped his hands together to pray, he said, for patience with MPs, few of whom turned up at the special session.

"If those shameless people don't approve the plan of territorial control, we'll summon you here again [next] Sunday," he told supporters in a fiery speech outside, as he left the building.

Politicians were due to meet on Monday (local time) to discuss the President's proposals, Congress president Mario Ponce said, in a possible sign of de-escalation.

Critics warned of a looming constitutional crisis, however.

Human Rights Watch, a US-based group, called the incident "an exhibition of brute force" and said the Organisation of American States should urgently meet to discuss the situation.

Nayib Bukele said Salvadorans had a legal right to insurrection in these kind of situations. ( AP: Salvador Melendez )

Mr Bukele won office last year after a savvy social media campaign feeding off popular discontent with two parties that ruled the Central American country from the end of a civil war.

Channelling that same frustration with traditional parties, he attacked Congress for dragging its feet on the approval of a $US109 million ($162 million) multi-lateral loan he has sought to equip police and soldiers to fight crime.

His Cabinet called the special session after Mr Bukele said Salvadorans had a legal right to insurrection in such situations, calling for protests and briefly removing politicians' security protection details.

The President's move to pressure MPs was backed by defence minister Rene Merino Monroy and police director Mauricio Arriaza Chicas.

Although the murder rate in El Salvador has declined steeply since Mr Bukele took office, authorities continue to battle gangs that control vast territory in the country.

In a statement, the United Nations High Commissioner for Human Rights called for "dialogue and full respect for democratic institutions to guarantee the rule of law, including the independence of the branches of public power".

Reuters