Ali Ahmed Jarallah was executed on Sunday in the courtyard of the main Sanaa prison in the presence of his family, judges and lawyers, judicial sources said, after President Ali Abdallah Saleh upheld the death penalty against him.
Jarallah, an imam or prayer leader at a mosque in Sanaa, was convicted of the murder in 2002 of Jarallah Omar, the deputy leader of the Yemeni Socialist Party, and his sentence was upheld by an appeals court in April.
Jarallah said in a confession in July 2003 that he had acted alone in shooting Omar at a party conference in Sanaa the previous year.
He claimed that the killing was part of a jihad, or holy war, against converts to Christianity and infidels.
Six other people were jailed for between three and 10 years for belonging to a gang that the cleric allegedly created to murder socialists, Arab nationalists and converts to Christianity.
Jarallah had been a member of Yemen's Islamist Al-Islah Party, but quit the movement shortly before the killing, complaining that it had gone soft on Westerners and minority Islamic sects.
He was accused by the court of using his mosque to recruit youths for attacks and of "sowing sedition" in Yemen, one of the poorest countries in the world.
The death sentence against him was carried out four years after the execution in 2001 of Abul Hassan al-Mezar, the head an extremist group in south Yemen who was convicted of murdering four tourists in 1998.
The Socialist party governed southern Yemen before it was unified with the north in 1990.