One of Bangladesh's most prominent government critics went missing on Monday after calling his wife to tell her he feared he would be killed, police said.
The wife of Farhad Mazhar, a well-known poet, writer and dissident, told police he telephoned early on Monday to tell her he was being taken away by a man and feared he would be killed.
Shortly afterwards, she received a ransom demand for her husband.
"His wife sent a man to the (police) station who told us Mazhar called few hours later after he left and said that he was being taken away and would get killed," said duty officer Aleya Akhter.
"After a while someone called his wife from the same phone and asked a ransom of 3.5 million taka ($43,317)."
The writer's family has filed a complaint with the police.
The country's elite Rapid Action Battalion (RAB) force were conducting a house-by-house search operation for Mazhar in the southwestern city of Khulna, 269 kilometres (167 miles) from the capital, after they claimed to have traced "signals" from the poet's mobile phone.
But hours later, there was no trace of Mazhar.
CCTV footage of Mazhar's neighbourhood in the capital Dhaka showed him walking away with a man early in the morning.
The 69-year-old is a supporter of the country's main opposition Bangladesh Nationalist Party (BNP).
BNP officials have said tens of thousands of their activists and supporters have been arrested by the government since 2014, when the party boycotted a controversial general election over fears it would be rigged.
There have also been allegations of so-called enforced disappearances -- where authorities detain people unofficially and hold them in secret jails.
"Far too many people have gone missing in Bangladesh over recent years without any further news of their fate. The government must end impunity for these abuses," Biraj Patnaik, South Asia Director of Amnesty International, said.
The rights watchdog released a statement urging the authorities to "make every effort to locate" Mazhar, "bring him back to safety and hold the perpetrators accountable".
Kidnapping for ransom is relatively rare in Bangladesh, and rights activists urged the government to properly investigate Mazhar's disappearance to prove it was not involved.
"Such incidents happen repeatedly and the government's silence is deplorable," said Nur Khan Liton, a prominent rights activist.
Rights group Ain O Salish Kendra says 39 people have gone missing in Bangladesh so far this year including top political and business figures.
Mazhar had on Sunday taken part in a press conference to protest a spate of lynchings of Muslims in India, a key ally of the current regime in Bangladesh.