A spokesman for Bahrain's Information Ministry told Aljazeera.net that four protesters were arrested while demonstrating in a prohibited zone.
According to rights activist Nabil Rajab, about 50 unemployed were demonstrating peacefully near the royal court when police attacked them.
"They beat them and arrested more than 30 of them," Rajab said.
He also told journalists that Abd al-Hadi al-Khawaja, head of the banned Bahrain Centre for Human Rights, was beaten and arrested, too.
But the ministry spokesman said the protesters were told demonstrations outside the royal court were prohibited and that police had asked them to move on.
"There were four separate protests going on this morning, only one of these chose to demonstrate in a place where this is prohibited. The other three were totally legal.
"I would add that 30 people were not arrested, but rather four, though some were forcibly removed from the royal court site," the spokesman said. The spokesman was unable to say if al-Khawaja was one of the four in custody.
The activist did not answer calls made to his mobile phone.
Bahrain is the least wealthy of the oil producing Gulf states, with high unemployment and a history of political tension.
The government says unemployment in the Gulf's banking hub is about 15%, but economists put it at closer to 20%.
Last year, al-Khawaja was pardoned by Bahrain's King Hamad bin Isa al-Khalifa after being sentenced to a year in prison for publicly blaming the prime minister for the country's economic woes.
Al-Khawaja's detention had led to demonstrations last year in which police clashed with and arrested protesters.
King Hamad has tried to accommodate opposition demands for greater reform.
Bahrain, a close US ally and home to the US Navy's Fifth Fleet since 1947, has been held up by Washington as a model of a more open and democratic society to be followed by other Arab states.