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 fervor.
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 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 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.
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 dismantling the security forces, prosecution of state officials for abuses and the release of 450 jailed activists, religious leaders and others.
The rights center's president, Nabeel Rajab, urged the king to "avoid the fatal mistake committed by similar regimes in Tunisia and Egypt" and not try to crush planned protests Monday. He warned further pressures by authorities could push the country into "chaos or bloodshed."
On Friday, hundreds of Bahrainis and Egyptian nationals went out in the streets chanting and dancing near the Egyptian Embassy in Manama moments after Hosni Mubarak stepped down as president. Bahraini authorities quickly set up roadblocks to contain the crowds.
The chances for confrontation in Bahrain have been further elevated by the ongoing trial of 25 Shiite activists - including two charged in absentia - accused of plotting against the state. The detainees have alleged police torture and being made to sign forced confessions, but the court has moved ahead with the proceedings. The next session is scheduled for Feb. 24.
Bahrain's Al-Wasat newspaper reported Sunday that one of the suspects had a heart attack while in custody and was hospitalized.
Opposition groups in Kuwait had called for an anti-government protests last week, but shifted the date to March 8 after the resignation of the country's scandal-tainted interior minister.