The opposition is now seeking to overturn the president's decree extending the terms of 25 top magistrates and civil servants, which they say is intended to help him secure his re-election.

They argue the decree violates the Nicaraguan constitution, and say that they have gathered the necessary 47 votes to overturn it.

"What they are showing is their fear of the majority being in power," opposition lawmaker Eduardo Montealegre said of the protestors.

Opposition leaders have said they will return to parliament on Thursday [Reuters]

"They are scared that we say 'no' to [Ortega's] re-election."

Opposition leaders have insisted they will return to parliament on Thursday, despite the blockade, setting the stage for a possible showdown and raising fears of further violence.

In Washington, Jose Miguel Insulza, the head of the Organisation of American States, expressed his "deep concern" over the crisis and called for calm.

He stressed the need to "resolve political differences, particularly those between state representatives, through dialogue."

Ortega led the 1979 Sandinista uprising that ousted the regime of US-backed dictator Anastasio Somoza and served as president from 1985-1990.

He was re-elected in 2006.