MANAMA (Reuters) - Bahrain has released 23 Shias accused of trying to topple the island's Sunni Muslim monarchy and who were among what the government said on Wednesday was a total of 308 prisoners freed on the orders of the king. The prisoner release was a further concession to the mainly Shia 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 preceded the expected return to Bahrain of Hassan Mushaimaa, leader of the hardline Shia Haq party, one of two people tried in absentia for his part in the alleged coup plot. The government said some of the released prisoners had alleged mistreatment in custody and it pledged to investigate. "All who allege mistreatment will be contacted by the Ministry to detail the specific allegations, so that investigations can commence immediately," it said in a statement, without specifying which ministry. One of the 23 freed in the alleged coup plot said he and others had been tortured. "In the first week they tortured us to pick up the information," Ali Abduleman, a Bahraini blogger, told Reuters. "(Once) I had to stand for five days." Mohammed al-Tajer, a lawyer for the 23 activists, said it was not clear if they 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. Opposition leaders welcomed the prisoner release. "Allowing the people to protest and releasing those people are positive moves," said Ibrahim Mattar of the main Shia Wefaq party. 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. Majority Shias have long complained of discrimination in Sunni-ruled Bahrain, which is a close US and Saudi ally. King Hamad bin Isa al-Khalifa was in Riyadh to welcome home Saudi Arabia's King Abdullah from an extended absence due to ill-health, a sign of tight ties between the Bahraini royals and their far stronger Saudi counterparts. Saudi Arabia is concerned unrest in Bahrain might spread to its own Shia minority. Bahrain's 70 percent Shia majority wants to overhaul a system where parliament has little power and policy remains the preserve of an elite centred on the royal al-Khalifa family. The al-Khalifa dynasty has ruled Bahrain for 200 years, and the family dominates a cabinet led by the king's uncle, who has been prime minister since independence from Britain in 1971. Before the prisoner release, Bahrain's rulers had agreed to allow peaceful protests and had offered dialogue on reform. "The main point we are waiting for is the initiative for political reform. Until now they didn't promise anything," Mattar said. "If they don't say it, we are wasting our time." YOUTHS EMERGE FROM JAIL Protesters had demanded the release of political prisoners in Bahrain, where seven people were killed and hundreds wounded in demonstrations last week. In the first releases, late on Tuesday night, two dozen relatives waited at the jail for teenaged prisoners, who emerged one by one with solemn, unshaven faces. Some waved victory signs. One waiting mother tossed a sprig of basil in the air to welcome her freed son, a Reuters photographer said. The 23 men jailed for the coup plot, who include some Shia clerics, were put on trial in October after a broad security crackdown on some Shia opposition groups in August. US Secretary of State Hillary Clinton praised Bahrain's king and crown prince for freeing political prisoners, allowing peaceful demonstrations and offering talks with the opposition. "These steps will need to be followed by concrete actions and reforms," she told reporters on Tuesday, warning that "there is no place for violence against peaceful protesters." Bahrain is a small but strategic US ally that hosts the US Navy's Fifth Fleet in a region overshadowed by fears of Iran's nuclear ambitions.