Counterterrorism officers are preparing a list of extremist speakers making provocative speeches online and at public gatherings.

Seven individuals have already been identified and the list is being updated, officers said.

The number of militant acts has fallen, thanks to crackdowns, but the threat of radicalisation is on the rise, said Saiful Islam, deputy commissioner of Counter Terrorism and Transnational Crime (CTTC) unit of police.

"The list we have been making contains the names of several leaders of Hefajat-e-Islam. We are working to bring to book the extremist speakers who are popular among militants," he said.

The speakers will be accused under the anti-terrorism act as "instigators".

The CTTC has an online monitoring process, he said, "But radicalisation cannot be stopped by monitoring alone. We have taken steps to take action."

Saiful said the move comes after a 20-year-old man arrested on Wednesday told officers that he was taken to militancy after watching online videos and had plans to attack the national parliament.

Al Sakib was arrested in the capital's Sher-E-Bangla Nagar area around 7:45pm, the officer said, adding that the suspect is a member of the Al-Qaeda inspired militant outfit Ansar Al Islam.

Police found a black flag, a black handkerchief, a large knife and a mobile phone in his possession.

Sakib told police that when he contacted the speakers, they inspired him to make a terrorist attack through forming a group of like-minded individuals.

He sent invitations to people on messaging applications and had the large knife made by a local blacksmith.

He reached Dhaka from Sirajganj on Wednesday with a plan to gather 313 individuals and die in police firing while attacking the national parliament, said DC Saiful Islam. Thankfully, no one responded to his invitation.

Around 10:30am yesterday, police arrested Ali Hasan Osama, 27, in Rajbari on charges of advising Sakib to be involved in terrorist acts, Saiful said.

Osama is one of the speakers who had inspired Sakib.

CTTC officials said Osama is popularly known as "Banglar Osama".

"Osama has assumed the role of being the spiritual leader of Ansar Al Islam after the arrest of Mufti Jasim Uddin Rahmani in 2013. Osama had written between 40 and 50 instigating books in the last five years and we kept him under surveillance for the last two years."

Rahmani was sentenced to five years in prison in connection with the murder of blogger Ahmed Rajib Haider.

Saiful said many in Osama's large base of followers became radicalised by watching his speech on YouTube, and Telegram.

Earlier, a Bangladeshi citizen arrested in Singapore and two others in India told law enforcers that they were radicalised by watching Osama's online speeches, said the officer.

Osama used to meet 50 to 100 followers everyday at different mosques and houses in Rajbari.

Mufti Harun Izhar, former education and cultural affairs secretary of Hefajat, is also identified as an inciter, officers said.

Izhar was arrested by Rab on May 2 on charges of his involvement in the violence in Hathazari on March 26. He is an adviser of the Shirajganj madrasa where Sakib studied.

Izhar will also be accused of militancy as he is an active organiser of banned militant outfit Harkat-ul-Jihad-al-Islami, a CTTC source said.