Moderate leaders of the Democratic Unity (MUD) coalition broke off dialogue this week, saying government officials were insulting them and rebuffing requests for releases of opposition-linked prisoners.

Unrest has flared again in Caracas in recent days, with more than 100 youths arrested during violent clashes with security forces and attacks on government buildings.

Since anti-government protests began in February, 42 people have been killed, more than 800 injured, and about 3,000 arrested, of whom more than 200 remain behind bars.

"We don't accept blackmail from anyone," Maduro said during a ceremony with Palestinian President Mahmoud Abbas in Caracas.

"They (the MUD) say they want immediate results. The dialogue itself is a positive result. What are they looking for? In private, they've said things that are impossible."

The opposition is particularly pressing for the release of former Caracas police commissioner Ivan Simonovis, who is serving a 30-year prison term for his involvement in a 2002 coup that briefly ousted Hugo Chavez.

Foreign ministers from Brazil, Ecuador and Colombia, who have been mediating the talks, were due to return on Sunday, but may now find themselves shuttling between the two sides.

"It would be very sad if they abandon the table, but the country would carry on, no one's going to stop working," Maduro said. "There would, though, be national repudiation of their anti-democratic attitude ... I urge sense and reflection."

Maduro, a socialist like the late Chavez, has cast the protest movement as a U.S.-supported coup plan, while foes say months of rallies are the product of economic hardship and repression.

Protest numbers have dropped in recent weeks, but a rump of masked youth activists still take to the streets near daily, sometimes setting up barricades and burning vehicles.