Bahraini police and protesters have clashed in several areas of the capital, Manama, in some of the worst unrest for weeks.
Security forces used tear gas and rubber bullets to battle protesters who had managed to blockade main roads into the financial district.
Clashes were also reported in the city's main square and university.
Mainly Shia protesters, inspired by the uprisings in Egypt and Tunisia, want the reform of the Sunni monarchy.
They have threatened to march on the royal palace if their demands are not met.
Police killed seven protesters last month at the height of the unrest, but since then, clashes with the security forces have been rare.
But on Sunday - which is a normal working day in many countries in the region - clashes broke out in three areas of Manama.
First, protesters blocked roads into the financial district, and were tackled by police using tear gas.
"No-one was able to go to work today. Thugs and protesters were blocking the highway," one worker, Sawsan Mohammed, told the AP news agency.
Then security forces moved to try to clear protesters from Pearl Square, which has been largely occupied by demonstrators for a month.
They fired tear gas and rubber bullets, but witnesses said thousands of protesters swarmed into the square and eventually forced the police to retreat.
There were also clashes between pro- and anti-government demonstrators at Bahrain University.
Bahrain's royals renewed their offers of dialogue, but moments later a speaker at Pearl Square said "no dialogue, no dialogue" to cheers from the crowd.
Bahrain - home to the US Navy's Fifth Fleet - is the first Gulf country to be thrown into turmoil by the wave of unrest sweeping the Arab world.
The US called for the government to have "peaceful and meaningful dialogue with the opposition rather than resorting to the use of force".
The UK government, which is warning its citizens to stay away from Bahrain, said there were fresh reports that Saudi soldiers were about to enter the country.
Shia account for about 70% of the country's 525,000 people, but the country has been ruled by a Sunni monarchy for hundreds of years.