A prominent human rights activist predicted "chaos and bloodshed" if attempts are made to crush the demonstrations.
The tiny kingdom of Bahrain is among the most politically volatile in the gulf and holds strategic value for the West as the home of the U.S. Navy's 5th Fleet. Bahrain's Shiites - accounting for nearly 70 percent 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 Monday - the anniversary of the promulgation of Bahrain's 2002 constitution, which brought some democratic 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 after a wedding ceremony. An Associated Press photographer saw several people injured and others gasping from tear gas.
Security forces set up checkpoints around Shiite villages and throughout the capital, Manama, to monitor people's movements. Units also patrolled shopping 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 multimedia service that includes social media applications aimed at greater outreach.
It's unclear, however, whether activists and rights groups will be satisfied with the proposed changes after facing widespread blocks on Web sites and blogs.
Last week, Bahrain's king, Hamad bin Isa al-Khalifa, granted each Bahraini family the equivalent of nearly $2,700 in an apparent bid to calm tensions.
But the demands go deeper than economics.
In an open letter to the king, the independent Bahrain Center for Human Rights called for wide-ranging reforms to avoid a "worst-case scenario," including the dismantling of the security forces, the prosecution of state officials for abuses and the release of 450 jailed activists, religious leaders and others.
The center's president, Nabeel Rajab, urged the king to "avoid the fatal mistake committed by similar regimes in Tunisia and Egypt" and advised against trying to crush planned protests. He warned that further pressures by authorities could push the country into "chaos or bloodshed."
- Associated Press