Bangladeshi police can tap phone calls they believe are suspicious under a law which came into effect on Monday.
The move is part of efforts to crack down on Islamist militants who are blamed for a series of deadly bomb attacks, the government says.
A spokesman for President Iajuddin Ahmed said he had signed into law an ordinance from the cabinet on Sunday.
At least 25 people have been killed in a series of bombings across Bangladesh since August this year.
The decision to give security forces powers to monitor phone calls was taken by the cabinet last week. Parliament is not in session.
Abuse fears
Home affairs minister Lutfuzzaman Babar says the Islamist network is maintained through mobile phones.
"If the law is applied on a wholesale basis, it will certainly curtail people's privacy "
"We know Islamist leaders use 20 to 30 different temporary cell phone numbers to guide the bombers... something we are going to crush soon," he told Reuters news agency last week.
More than eight million people in Bangladesh use mobile phones, with nearly another million using land lines.
The new law has prompted concerns it will lead to people's rights being infringed.
"If the law is applied on a wholesale basis, it will certainly curtail the people's privacy and violate their fundamental rights," Supreme Court lawyer Shahdeen Mallik told the BBC.
Law minister Modud Ahmed said those who were innocent had nothing to fear.
To prevent misuse only a minister or a secretary would have the authority to give permission to tap a telephone, he said.
Officials said the ordinance would be taken to parliament for ratification when it next meets.
An ordinance usually remains in force in Bangladesh for six months until approved by the parliament.
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