Nearly 200,000 security personnel are being deployed in northern India ahead of a court ruling on the long-running Ayodhya religious dispute.
Helicopters are keeping watch overhead and authorities have urged calm amid fears the ruling could spark unrest.
The Allahabad High Court will decide who owns land where Hindu mobs tore down a 16th Century mosque in 1992.
Hindus claim the site of the Babri Masjid is the birthplace of their God, Ram, and want to build a temple there.
The destruction of the mosque led to widespread rioting between Hindus and Muslims in which some 2,000 people died.
It was some of the worst religious violence since the partition of India in 1947.
The high court ruling in the Ayodhya case is due to be announced in the city of Lucknow, in Uttar Pradesh state, on Thursday at 1550 local time (1000 GMT).
Home Minister Palaniappan Chidambaram said on Wednesday that there would be 190,000 security personnel on duty in the state for the ruling.
Mobile phone text message services have been temporarily banned in an attempt to prevent anyone from inciting violence and a media watchdog has appealed to broadcasters to avoid showing inflammatory images, the BBC's Sanjoy Majumder in Delhi reports.
"The central government has taken adequate measures and has deployed adequate forces to assist state governments in maintaining peace," Mr Chidambaram told a press conference in Delhi.
"I once again appeal to the people to maintain peace."
Mr Chidambaram said all state governments had been "advised to be firm and maintain public peace and order and I'm sure they will do it".
Correspondents say the authorities are anxious as the legal decision could have potentially explosive consequences.
An appeal for peace, signed by Prime Minister Manmohan Singh, has appeared in several Indian newspapers urging people to respect the rule of law and abide by the court order.
Correspondents say the Ayodhya ruling could not have come at a worse time for the authorities - they already have their hands full dealing with security preparations for the Delhi Commonwealth Games which begin on Sunday.
Moreover, many troops are engaged in fighting Maoist rebels across vast tracts of India and the worsening situation in Indian-administered Kashmir has added to security problems.
The court ruling was due last Friday but the Supreme Court deferred the decision, saying it wanted to give Hindus and Muslims more time to resolve the dispute amicably. On Tuesday it said the high court could proceed.
At stake is whether the disputed site in Ayodhya should be given to the Hindu community to build a temple or returned to the Muslim community to rebuild the Babri Mosque.
Correspondents say Thursday's ruling is unlikely to be final and expect the judgement to be appealed.