Security forces have clashed with protesters trying to march towards the Bahraini capital's symbolic Pearl Roundabout to mark the third anniversary of the country's anti-government protests, witnesses said.
Protesters, who gathered in several Shia villages on Friday to walk towards the roundabout, where demonstrators camped out for a month in early 2011 before being violently dispersed by troops, were met with tear gas and bird shot, according to witnesses who spoke to AFP.
Several demonstrators were wounded as forces beefed up security around the area and dispersed protesters emerging from nearby villages, sources said.
"Down with Hamad," protesters, some wrapped in white shrouds and others covering their faces, chanted, referring to the king.
"We will never surrender," they shouted as police helicopters hovered overhead and police forces manned checkpoints outside Shia villages, witnesses said.
Earlier on Friday, Bahrain Centre for Human Rights (BCHR) announced that they had documented 36 arrests while seven were released from jail.
The opposition has called for three days of protests to mark the anniversary as it seeks to give new momentum to its campaign for the ruling al-Khalifa family to surrender its grip on all key cabinet posts in favour of an elected government.
The Pearl Roundabout and its popular namesake monument, which were a symbol of the 2011 erallies, was later demolished in March the same year and the site remains heavily guarded ever since.
The Ministry of Interior said a police bus was struck in a bomb attack in the Shia village of Daih, without reporting casualties.
The ministry said on its official Twitter account: "Terrorist explosion in Daih damaged a bus transporting police personnel."
Witnesses told AFP that a loud explosion was heard and police cordoned off the area.
A ministry official near the scene of the blast told reporters that three police officers were injured in the explosion. He insisted on anonymity because he was not authorised to speak publicly. However, pictures posted by the ministry on Twitter showed the bus slightly damaged on its sides.
The "February 14 youth coalition" had called for the march to Pearl Roundabout. Bahrain announced in June that it had arrested leading members of the cyber-group, accusing it of links to Iran.
Amnesty International on Thursday condemned Bahrain's "relentless repression" of dissent and said it feared a violent crackdown on the anniversary demonstrations.
The International Federation for Human Rights urged Bahrain "to take immediate measures to restore the rule of law, to put an end to ongoing human rights violations."
Six opposition groups led by Al-Wefaq called in a statement on Friday for an "overall political process to rebuild the state... free elections.. and an independent judiciary."
In a speech published on the day, King Hamad stressed his "commitment to complete reform in accordance with our circumstances, national interests, identity and values".
Previous rounds of national dialogues have failed to significantly close the rifts between the two sides and the opposition is demanding amnesty for what they claim are more than 3,000 political prisoners held in Bahraini jails. Crown Prince Salman bin Hamad al-Khalifa met top opposition leaders last month to once again revive the talks.