Rahul Gandhi said he stood "shoulder to shoulder" with Mamata Banerjee. (File)

Congress president Rahul Gandhi on Sunday expressed solidarity with West Bengal Chief Minister Mamata Banerjee, who sat on protest after a team of Central Bureau of Investigation (CBI) visited Kolkata police chief Rajeev Kumar's house.

The CBI officials, who had come to question Mr Kumar on two Ponzi schemes, were temporarily detained by the police.

"I spoke with Mamata-di tonight and told her we stand shoulder-to-shoulder with her. The happenings in Bengal are a part of the unrelenting attack on India's institutions by Prime Minister Narendra Modi and the BJP. The entire opposition will stand together and defeat these fascist forces," Mr Gandhi tweeted.

Rajeev Kumar -- who headed a Special Investigation Team probing the Saradha and Rose Valley Ponzi scams of 2013 -- was asked to help the investigation after several documents allegedly went missing. He has received several summons for questioning. This time, a section of media reported that he could be arrested for evading questioning and was "absconding".

Mamata Banerjee is staging a sit-in in central Kolkata's Metro Channel area

When the CBI team on Sunday approached Mr Kumar's house, officers of the Kolkata police whisked them away and temporarily detained them in a police station. They were released later.

After the showdown, Ms Banerjee staged a sit-in in central Kolkata. She was accompanied by Mr Kumar. "How dare you come to a police commissioner's home without a warrant? Mr (Ajit) Doval is doing whatever PM wants him to do, he is the one giving all the instructions to CBI," Ms Banerjee said, referring to the National Security Advisor.

The leaders of several prominent opposition parties have expressed solidarity with Ms Banerjee. While AAP's Arvind Kejriwal, National Conference leader Omar Abdullah spoke to Ms Banerjee on phone, Samajwadi Party chief Akhilesh Yadav, DMK's MK Stalin, TDP's N Chandrababu Naidu tweeted in her support.