Banerjee, who arrived here earlier in the day, met Andhra Pradesh Chief Minister Chandrababu Naidu before attending the Opposition meeting, in which 21 parties were present.
Sources said the two leaders discussed their plan of action and issues to be raised in the Parliament.
In fact, sources said the alleged breakdown of institutions, like the CBI and RBI, under the Narendra Modi government were discussed at length.
After the Opposition meeting, Banerjee met Delhi Chief Minister Arvind Kejriwal and Aam Aadmi Party leader Sanjay Singh. The two sides, sources said, discussed ways to stop the Bharatiya Janata Party in the 2019 general election.
They also agreed on common points which they decided to raise in the Parliament session that starts Tuesday, sources said.
The Trinamool Congress supremo also met senior Congress leader Ahmed Patel.
Banerjee, who has been vocal against the Centre's economic policies like demonetisation and the implementation of the Goods and Services Tax (GST), also hit out at the BJP-led government over the resignation of RBI governor Urjit Patel.
"This has never happened before. This is unprecedented. We are deeply concerned. The RBI is the custodian of public money. The credibility of all institutions is being destroyed. This is a financial and economic emergency," she said in a tweet.
"BJP is behaving like a dictator. Political emergency already exists everywhere. They are threatening every political party and so many leaders sent to jail. If the RBI governor also has to resign, its a matter of grave concern for the country. With this, financial emergency has also started," she said.