The activist, Sayed Yousif al-Muhafda, from the Bahrain Centre for Human Rights, had been arrested during an anti-regime protest rally last month, Amnesty International said.
The released activist’s lawyer said at the time that Muhafda was there to observe the march in Manama, adding that he was already being investigated on charges of spreading false news on Twitter.
Bahrain’s Shiite Muslim majority along with its Sunni brethren launched pro-democracy protests in February 2011, demanding a democratically –elected government lead the tiny Persian Gulf island nation.
But, the ruling al Khalifa regime promptly launched a brutal crackdown on the protests and called in Saudi and UAE troops from its neighbors to help silence the voice of dissent.
The regime has stepped up efforts to end the unrest in recent months and several activists have been arrested or jailed for organising or taking part in protests.
Earlier this month, a decision by an appeals court to uphold prison terms for 13 protest leaders drew international condemnation. Activists said the ruling showed Bahrain's judicial system was flawed and unable to protect basic rights.
The opposition, meanwhile, says little progress has been made towards its demand for reform, including a parliament with full powers to legislate and form governments. Many Shiite Muslims also complain of political and economic marginalization.
MOL/