They said the protesters gathered on Friday at a rallying point renamed "Freedom Square" in al-Muqsha village, about seven kilometers (four miles) west of Manama.
They waved Bahraini flags and banners calling for the overthrow of the ruling Sunni al-Khalifa family. "Down with Hamad," some banners read, referring to King Hamad.
Organizers said the rally would last for a week and take place every afternoon until midnight.
"Bahrainis will not go back on their demands," a leading opposition figure from Bahrain’s main Shiite opposition formation al-Wefaq, Abduljalil Khalil, told the gathering.
The Shiite-led opposition demands constitutional changes that would reduce the power of the ruling dynasty. Tensions have remained high in Bahrain since a deadly crackdown last year after a month of Manama street protests.
