The prisoner release was a further concession to the protesters who took to the streets last week to demand a constitutional monarchy and an elected government, emboldened by a surge of popular unrest across the Arab world.

It also preceded the expected return to Bahrain of Hassan Mushaimaa, leader of the Haq party and one of two people tried in absentia for his part in the alleged coup plot.

Mohammed Al-Tajer, a lawyer for the 23 activists, told Reuters that about 250 prisoners had been released. Most were detained as part of a crackdown launched on some opposition groups last August and during subsequent protests.

The opposition describes youngsters arrested during clashes with security forces as political prisoners.

Tajer said detainees picked up in last week's confrontations between security forces and protesters had been freed earlier. He said it was not clear if the 25 charged in the coup plot had received royal pardons or if the case could be revived. "If this is just a suspension of the charges, they might bring up the case again or file other charges. We're waiting for an official statement," he said.

Welcoming the releases, Ibrahim Mattar of Al-Wefaq party said: "Allowing the people to protest and releasing those people are positive moves." Opposition groups were waiting for the royal family to accept the principle of a constitutional monarchy before they would enter into a dialogue offered by the king, he added.

"The main point we are waiting for is the initiative for political reform. Until now they have not promised anything," Mattar said. "If they don't say it, we are wasting our time."