Bahrain Mirror (Reuters): The trial in Bahrain of prominent rights activist Nabeel Rajab was postponed until Aug. 2 on Tuesday and a request for his release rejected, his lawyer said, hours after 26 rights groups jointly called for him to be freed.

Bahrain has made no public statements on Rajab's case, but his lawyer has said charges against him relate to anti-government tweets he allegedly published last year, including one accusing the security forces of torturing detainees.

Rajab was a leading figure in a 2011 pro-democracy uprising led by Bahrain's Shi'ite Muslim majority, and he has been repeatedly detained.

His latest arrest comes during what rights groups say is an escalating crackdown on Bahrain's opposition.

The trial was postponed until Aug. 2 and a request for his release by the defence was turned down, his lawyer Mohammed al-Jishi wrote on Twitter without elaborating.

Shi'ites in Bahrain complain of discrimination from the Sunni-led government, which denies this.

In recent weeks, a court shut down a main Shi'ite opposition society and the interior ministry stripped the spiritual leader of the island's Shi'ites of his citizenship.

Violence between Shi'ites and security forces has continued on the island since the 2011 uprising was put down with the help of Saudi Arabia.

Hours before Rajab's hearing was due to take place, 26 human rights groups put out a joint statement.

"We remind the Bahraini government of its obligation to preserve the right to free expression," the campaign groups, including Human Rights First and Physicians for Human Rights, said.

"We reiterate repeated calls by United Nations officials, and others in the international community, to immediately release Rajab," they added.

Rajab was taken to hospital for a day late last month for what his supporters said was an irregular heart beat, but a spokesman for the prisons authority described his condition as "normal," in a statement e-mailed to Reuters.

Bahraini rights groups said authorities on Monday freed Ibrahim Sharif, former head of the secular Waad party, after he served a year in jail for what state news agency BNA said was publicizing "hatred of the regime".

Government officials deny systematic abuses of human rights and have accused the opposition of stirring sectarian hatred in the kingdom and serving the interests of their rival, Shi'ite Iran.