Venezuela's opposition has ruled out discussions with President Nicloas Maduro on his plan for an elected assembly to draw up a new constitution, vowing instead to continue protests for early elections.

Opposition leader Henrique Capriles formally announced on Sunday that the Democratic Unity Roundtable (MUD) would boycott a meeting with Maduro at the presidential palace on Monday.

"We cannot take part in a fraudulent process," Capriles said, appearing at a news conference with other MUD leaders.

"We have a constitution, and the government cannot repeal it by act of force."

The opposition maintains that Maduro, whose leadership is rejected by seven out of every 10 Venezuelans, according to polls, is trying to avoid a general election - the main demand of the opposition protests since April 1.

Some analysts, however, said they fear the opposition is leaving the way open for Maduro to change the constitution at will, possibly cementing his grip on power.

Venezuela's worst economic crisis - What went wrong?

Tens of thousands of protesters have turned out for near-daily protests in Caracas for weeks, seeking to remove Maduro and put an end to a devastating economic crisis that has caused widespread food and medicine shortages.

At least 37 people have died in violent clashes, and hundreds more have been wounded or jailed.

Al Jazeera's Lucia Newman, reporting from Caracas, said riot police were out in force in the capital on Monday morning, and most highways and metros leading from the city's east to its centre were closed.

Maduro's plan for a new constitution has "further inflamed political passions" in Venezuela, she said.

The opposition views the plan as a coup d'etat and a power grab because the powers of the new constitutional body "will supersede all other powers of other institutions, especially the opposition controlled congress", Newman said.

Some analysts warned that MUD's decision to sit out that process could allow the Maduro government to unilaterally change the constitution.

"If the opposition maintains its position," political scientist Luis Salamanca said, "Maduro could change the very model of the state to perpetuate his grip on power."

"Do not leave the space free," he warned, adding: "This could be the last election in Venezuela."

OPINION: The end of autocracy in Venezuela?

Maduro, who was elected to replace the late Hugo Chavez in 2013, claims protesters are in fact seeking a coup with US support and harboring "terrorists" and "murderers" in their ranks.

"There are two paths: either there is peace or there is fascism. Either we rewrite the constitution or we bow to interventionism," he told aides by phone late Sunday, according to the AFP news agency.

"We're facing an armed insurgency," he told state television on Sunday.

Maduro's plan has drawn widespread international criticism.

The United States, Mexico, Brazil and Spain have said that any process to modify the constitution requires universal suffrage.