Former Spanish Prime Minister Felipe Gonzalez has arrived in the Venezuelan capital, Caracas, to help with the defence of jailed opposition politicians.
Mr Gonzalez will meet lawyers and families of the prisoners.
He postponed a trip in May when the trial of one of the opposition leaders, Leopoldo Lopez, was cancelled.
The Venezuelan Congress has classified him as a "persona non grata".
Two other former leaders, ex-presidents Andres Pastrana of Colombia and Jorge Quiroga of Bolivia, have also offered their help to members of Venezuela's opposition.
In a visit to the country last week they were forbidden from visiting Mr Lopez, Daniel Ceballos and Antonio Ledezma, opponents of the government of President Nicolas Maduro who are all in jail.
President Maduro has described the interest of the foreign leaders in Venezuelan affairs as part of a hostile campaign against his government and the country.
Mr Lopez is on hunger strike in a military prison outside Caracas while he awaits his trial.
He is accused of being responsible for the violence during a series of anti-government protests last year in which 43 people died, some of them police officers.