Thousands of police and soldiers are patrolling the streets of Bangladesh's capital, Dhaka, ahead of a major anti-government rally.
Schools and shops were closed as crowds of Bangladeshi opposition supporters gathered in Dhaka.
The Bangladesh Nationalist Party (BNP) says hundreds of its members have already been arrested.
The BNP wants the government to step down and hold new elections under a neutral caretaker administration.
The ruling Awami League has already rejected the idea. The next national election is due to be held by early 2014.
Under the previous system, at the end of an elected government's term, a caretaker administration would take over to ensure that elections were conducted in an impartial manner.
The BNP believes elections overseen by the incumbent government would not be free or fair and has said it will not take part in the next elections if the caretaker government system is not restored.
At least 15,000 policemen and border guards had been deployed in the capital, Dhaka's deputy police commissioner told the AFP news agency.
The BNP says hundreds of their supporters have been arrested in the last few days all across the country.
It also accused the government of preventing many more from joining the rally by cancelling bus and ferry services to Dhaka
But Dhaka police denied to AFP that any arrests were to do with the rally. The government says it is only taking precautionary measures to prevent any violence during the rally.
The BBC's Ethirajan Anbarasan in Dhaka cites fears that the rally could trigger more anti-government protests affecting the uneasy political peace of recent years.
Some warn that the political turmoil could threaten the country's already weak economy and drive away investors.