DUBAI, April 1 (Reuters) - Bahrain released a prominent blogger but detained several people, including a pro-opposition doctor, the latest in a series of arrests since the kingdom's crackdown on street protests, opposition sources said on Friday.
The tiny island's Sunni rulers have stepped up arrests of cyber activists and Shi'ites, with more than 300 detained and dozens missing since the crackdown on pro-democracy protests earlier this month.
It imposed martial law and called in troops from fellow Sunni-ruled neighbours, including Saudi Arabia, to quell the protest movement led mostly by the state's Shi'ite majority.
More than 60 percent of Bahrainis are Shi'ites and most want a constitutional monarchy.
Mattar Ibrahim Mattar, a member of Bahrain's largest Shi'ite opposition group, Wefaq, said the party's official arrest count was 329 by Thursday, but that the real number was likely to be over 400.
He said at least 20 people had been detained on Thursday and 31 were missing. It was unclear if those people were in hiding or had been abducted.
There have been several reports of missing people who have turned up dead days later, but activists say that many of their peers are also going into hiding to avoid arrest.
Prominent blogger Mahmood al-Yousif, who for years has promoted anti-sectarianism under the slogan "No Shi'ite, No Sunni, Just Bahraini", was detained on Wednesday and released late on Thursday.
"I'm back home now with my family. Everything is fine," he told Reuters by telephone. "I've been treated well enough. They investigated me but didn't find anything."
Opposition sources said Abdul Khaleq Al Oraibi, a doctor at Salmaniya Hospital, the kingdom's biggest, had also been detained.
Oraibi, who once considered running as a member of parliament for Wefaq, had been publicly critical of the lack of access for medics to wounded protesters.
The severity of Bahrain's crackdown, in which public gatherings are banned and security forces have been deployed at checkpoints, stunned Bahrain's Shi'ites and angered the region's non-Arab Shi'ite power Iran.
Saudi Arabia and other Gulf Arab states fearful of rising Iranian influence see Bahrain as a red line among the popular uprisings that have swept the region since January. (Writing by Erika Solomon; Editing by Nick Macfie)