A group of former world leaders has sent an open letter to the Venezuelan government expressing its concern "for the difficult social, economic and political situation" in Venezuela.
The group also urged the release of opposition leaders who were jailed for allegedly inciting violence.
The letter was signed by 26 ex-leaders from Latin America, Europe, Africa and Asia.
Venezuela has not yet responded to their appeal
The signatories are members of the Madrid Club, an independent organisation made up of former heads of state and government.
They include former Brazilian President Henrique Cardoso, the ex-leader of Mexico, Felipe Calderon, as well as former Polish President Aleksander Kwasniewski, Dutch ex-Prime Minister Ruud Lubbers and the former president of Nigeria, Olusegun Obasanjo.
In their letter, they call for the release of "all those citizens who are in prison today for having exercised their right to freedom of expression", including Caracas Mayor Antonio Ledezma and opposition leaders Leopoldo Lopez and Daniel Ceballos.
They go on to demand that former Spanish Prime Minister Felipe Gonzalez be allowed to travel to Venezuela to work with Mr Lopez's defence counsel.
They also claim their "right to appear in the oral hearing of Leopoldo Lopez", scheduled for mid-May.
Mr Lopez handed himself in to the authorities in February last year.
He is on trial on charges of inciting violence at mass protests which swept through Venezuela in the first half of last year.
Forty-three people from both sides of the political divide died during several months of protests.
Venezuela's National Assembly last week declared Mr Gonzalez "persona non grata" after President Maduro accused him of waging a "psychological war" with the aim of destabilising Venezuela.
Last month, President Maduro called Felipe Gonzalez a "lobbyist, who is getting a huge salary in euros for publicly joining the campaign against Venezuela, backing a coup against the country, a coup against me".
Felipe Gonzalez, who trained as a lawyer before becoming Prime Minister of Spain from 1982 to 1996, had announced earlier that he would join the team defending Mr Lopez and Mr Ledezma.
Mr Ledezma was arrested in February and charged with conspiracy. He is accused of backing an alleged plot to depose President Maduro.
Mr Ledezma, like Mr Lopez and Mr Ceballos, is a member of the opposition.
Their supporters say the charges against them are politically motivated, but the government argues they want to violently overthrow the democratically elected government of Mr Maduro.