A renowned NGO that was invited by Venezuelan officials to monitor Sunday's presidential poll has said the election "cannot be considered democratic".

The US-based Carter Center deployed 17 experts and observers to Venezuela after being asked to monitor the election by the National Electoral Council (CNE).

On Monday, the CNE - which is dominated by government allies - declared President Nicolás Maduro the winner, but the result has been disputed by the opposition which says voting tallies show its candidate, Edmundo González, has won by a wide margin.

In a statement released on Wednesday, the Carter Center said that it could not "verify or corroborate the results of the election declared by CNE".

The Carter Center also said that the CNE's failure to announce the detailed results by polling station "constitutes a serious breach of electoral principles".

It added that the CNE had "demonstrated a clear bias in favour of the incumbent [President Nicolás Maduro]" and accused the CNE of a "complete lack of transparency in announcing the results".

With its statement, the Carter Center has joined a long list of countries and organisations pressuring the CNE to release detailed voting data at the polling station level, among them the US, Brazil and the EU.

However, the statement by the Carter Center is likely to sting the Maduro government because its observers had been complimentary about the Venezuelan electoral system in the past.

President Maduro has often quoted a remark by the Carter Center's founder, former US President Jimmy Carter, who in 2012 said that "of the 92 elections that we've monitored, I would say the election process in Venezuela is the best in the world".

The disputed result announced by the CNE has trigged a wave of protests in Venezuela with thousands taking to the streets for a second day on Tuesday.

NGOs say there have been at least 11 deaths in protest-related violence and that dozens more have been injured.

Defence Minister Vladimir Padrino read out a statement on Tuesday branding the protests "a coup" which he said the armed forces would defeat.

More than 700 people have been detained, according to Venezuela's attorney general, who also said that two members of the security forces had died in the protests.

The opposition leader, María Corina Machado, has urged her supporters to remain peaceful even if provoked by the government and its allies.

On Tuesday, a close ally of President Maduro, National Assembly leader Jorge Rodríguez, called for the arrest of Ms Machado and Mr González, accusing them of leading a "fascist conspiracy".

The Costa Rican foreign minister later offered Ms Machado and Mr González political asylum, saying that his government "had been informed of arrests warrants" against the two.

Ms Machado thanked the Costa Rican government but said that it was her "responsibility to continue this struggle alongside the people".