Thousands of Shiite-led protesters calling for greater rights have streamed into an area outside the capital Manama.
Friday's rally is one of the largest demonstrations in weeks against the Gulf kingdom's Sunni rulers.
Some of the protesters carried tents, suggesting an attempt to occupy the site in A'ali, about 1.5 miles (three kilometers) southeast of Manama.
Others waved flags from nations where Arab uprisings have toppled regimes, such as Libya and Tunisia. Security forces did not make an immediate attempt to intervene.
Bahrain's majority Shiites began protests in February calling for a stronger voice in the nation's affairs. Sunni leaders have offered reforms, but refuse to give up control of top government posts.