Dozens of pro-government demonstrators marched to the offices of a Bahraini opposition party on Saturday and daubed the building with grafitti against majority Shi'ites and Iran, residents said.
They said "Down with Iran" and "Shi'ites get out" were among the slogans written on the offices of Waad, a secular party aligned with the largest Shi'ite opposition group Wefaq which was at the forefront of protests against the Sunni-led government this year.
US-allied Bahrain has accused Shi'ite power Iran of instigating unrest among Shi'ites in the Gulf Arab kingdom, an allegation Tehran denies.
"Police stopped them from entering. They dispersed after leaving pictures of King Hamad and the prime minister outside the Waad headquarters," a resident said.
"This building had been burned down twice, and we had just repaired it. So we were afraid that may be repeated. But police were there and nothing happened," said Radhi al-Musawi, Waad's deputy secretary-general.
Inspired by Arab Spring revolts in Tunisia and Egypt, thousands of mainly Shi'ite Bahrainis took to the streets in February and March demanding curbs on the power of the ruling Sunni Muslim Al-Khalifa family.
The protest wave was suppressed with the help of military forces brought in from Saudi Arabia and the United Arab Emirates. Bahrain hosts the US Fifth Fleet.
A government-appointed commission of international jurists found evidence of systematic abuses against detained protesters.
Bahrain has hired US and British police chiefs to lead the reform of security agencies.
There has been no progress in talks between the government and opposition groups on political reform and the Gulf Arab island state remains tense, with daily clashes between riot police and Shi'ite protesters.