Leopoldo Lopez handed himself in to the authorities on February 18 amidst anti-government protests. Poto: AFP

Mr Lopez has been in prison since last February on charges of inciting violence during mass anti-government protests held at the beginning of 2014.

On Sunday, Venezuelan President Nicolas Maduro said he would free Mr Lopez in exchange for the release of a Puerto Rican nationalist held in the US.

A US official said on Monday that the two cases could not be compared.

Lengthy sentence

During a televised speech on Sunday, President Maduro said that "the only way I would use my presidential powers [to release Mr Lopez] would be to put him on a plane to the United States, to leave him there, and that they hand over Oscar Lopez Rivera - man for man".

Lopez Rivera, 72, was convicted in 1981 of seditious conspiracy for seeking to secure Puerto Rican independence from the US.

In 1999, he rejected a conditional offer of clemency which then-US President Bill Clinton offered him and a dozen fellow members of the Armed Forces of National Liberation (FALN) armed group.

According to US officials, the FALN was involved in more than 100 bombings in New York, Chicago and other US cities.

Lopez Rivera was sentenced to 55 years. Another 15 years were later added to his sentence for attempting to escape from prison.

At the time of his clemency offer, President Clinton argued the sentences given to FALN members were out of proportion with their offences.

According to his lawyer, Lopez Rivera rejected the offer because it had not been extended to all jailed FALN members.

'No comparison'

US state department spokeswoman Jen Psaki said the case of Mr Lopez and Lopez Rivera could not be compared.

She also stressed that the US had "repeatedly requested the release of all political prisoners" and lamented that "President Maduro proposes to send into exile opposition figures instead of having a discussion about the real concerns and problems confronting Venezuela".

Leopoldo Lopez of the opposition Popular Will party handed himself in to the authorities on 18 February amidst a wave of anti-government protests.

The government says Mr Lopez was key in whipping up the protests in which more than 40 people from both sides of the political divide died.

Supporters of Mr Lopez say he is a political prisoner who is being punished for his opposition to the government of President Maduro.