The wide-ranging clampdown appeared directed toward Bahrain''s Shiite majority which had led the drive for Monday''s rallies and reflected the increasing worries of the Sunni rulers who have already doled out cash and promised greater media reforms in an effort to quell the protest fervour.
A prominent human rights activist predicted "chaos and bloodshed" if attempts are made to crush the planned demonstrations.
The tiny kingdom of Bahrain is among the most politically volatile in the Gulf and holds important strategic value for the West as the home to the US Navy's 5th Fleet.
Bahrain's Shiites accounting for nearly 70 per cent of the population have long complained of systematic discrimination by the ruling Sunni dynasty, whose crackdown on dissent last year touched off riots and clashes.
Shiite-led opposition groups and others have joined calls for the demonstrations on a symbolic day the anniversary of Bahrain's 2002 constitution that brought some pro-democracy reforms such as an elected parliament.
In Karzakan, a Shiite village in western Bahrain, riot police clashed with a small group of youths who staged a march following a wedding ceremony.
An Associated Press photographer saw several people injured and others gasping from tear gas. Security forces set up checkpoints around the Shiite villages and throughout the capital Manama to monitor people's movements. Units also patrolled malls and other key spots in a clear warning against holding the rallies, which have been the focus of social media appeals and text messages for more than a week.
One cartoon posted on a Bahraini blog showed three arms holding aloft a mobile phone and the symbols of Facebook and Twitter.
Bahrain's leaders, meanwhile, have stepped in with concessions to try to defuse the protests.
Government media monitors began talks Sunday with newspaper publishers and others to draft new rules to limit state controls. The official Bahrain News Agency, meanwhile, launched a new multimedia service that includes social media applications to seek more outreach.
It's unclear, however, whether activists and rights groups will be satisfied with the proposed changes after facing widespread blocks on websites and blogs.