Raids at the Social Democratic Party of India (SDPI) and Popular Front of India (PFI) offices and the residences of their office-bearers were carried across 10 states in what National Investigation Agency (NIA) described as the "largest ever" investigation process.
More than 100 PFI activists including top leaders were taken into custody.
The NIA in joint operation with Enforcement Directorate (ED) conducted the anti-terror raids. The NIA team detained a few key SDPI and PFI activists, seized documents, literature, computers, laptops and phones as well.
As per news reports, the raids were being planned for the past three months with the aim to break the backbone of the radical outfit. The search operation was conducted under the watch of NSA Ajit Doval.
PFI's name has often cropped up in cases related to terror funding, organisaing training camps, and radicalising youth.
The states where the raids were carried out included Uttar Pradesh, Kerala, Andhra Pradesh, Telangana, Karnataka and Tamil Nadu. In the non-BJP states, NIA team was given CRPF cover for safety purposes.
Activists of SDPI and PFI staged protests in front of their offices against the raid asking the NIA officials to go back.
In February last year, the ED filed its first charge sheet against PFI and its student wing Campus Front of India (CFI) on money laundering charges, claiming its members wanted to ''incite communal riots and spread terror'' in the aftermath of the Hathras gang-rape case.
Those named in the charge sheet include K A Rauf Sherif, national general secretary of CFI and a member of PFI; Atikur Rahman, national treasurer of CFI; Masud Ahmed, Delhi-based general secretary of CFI; journalist 'associated with PFI', Siddique Kappan and Mohammad Alam, another CFI/PFI member.
In the second charge sheet filed this year, the ED had claimed that a hotel based in the UAE hotel ''served'' as a money laundering front for the PFI.
(with inputs from PTI)