Dozens of people, including police officers have been injured in unrest in Bahrain, on the second anniversary of a Saudi-led intervention in the gulf island state.
The Bahraini interior ministry reported that "domestic terrorists" had blocked roads and burnt cars.
The main opposition group, al-Wefaq, said that at least 35 protesters had been hurt, three of them critically.
The interior ministry reported two police officers injured.
Some of Thursday's worst violence occurred in the towns of Sitra and Sanabis on the outskirts of Manama.
Cars were set ablaze and a major highway was temporarily blocked by protesters.
Witnesses reported heavy clashes as police used tear gas and stun grenades against youth armed according to the Interior ministry with "Molotov cocktails, iron rods and a number of homemade bombs."
Saudi Arabia led a Gulf Cooperation Council (GCC) force down a causeway that links the two countries on 14 March 2011.
The military intervention was intended to restore order after pro-democracy activists had peacefully taken control of an iconic landmark, Pearl Roundabout, in the capital Manama.
Saudi troops took up positions at key installations while police and the Bahrain Defense Force cleared the roundabout using force.
In the wake of the clearance more than 30 people died, hundreds were injured and thousands arrested. Several thousand more were dismissed from their jobs. The vast majority were Shia Muslim.
Bahrain has a Shia Muslim majority but is governed by a Sunni royal family. Shia Bahrainis have long complained of discrimination, poor housing and high unemployment.
The extent to which Shia were targeted by the government and its security forces after Pearl Roundabout was cleared served to fuel resentment.
As protests both peaceful and violent continued, King Hamad commissioned an independent enquiry. It found that police had used excessive force in subduing dissent. The King accepted the findings in November 2011 and promised change but demonstrations against the government have continued.
Opposition groups, led by al-Wefaq, have repeatedly called for all protest to be peaceful. But they say that the government is not committed to serious reform and is continuing to drag its feet.
That, says the opposition, is causing growing frustration as young masked Bahrainis take to the streets on a near nightly basis, many of them armed with Molotov cocktails. They routinely block roads with burning tyres, breeze blocks and rubbish bins.
The police have responded with tear gas, birdshot and stun grenades, coming under attack as they attempt to clear the barricades.