The police claimed that Disha Ravi, who was arrested on Saturday from Bengaluru, sent the "toolkit" to teen climate activist Greta Thunberg through the Telegram app, and also "coaxed her to act on it".
The toolkit is a Google document that police say was part of a Khalistani conspiracy to wage war against India by, among other things, instigating violence during the farmers' Republic Day tractor rally.
The move was quickly met with protests and criticism.
After Disha Ravi has been sent to police remand for five days, sources in Delhi Police said that more arrests are likely, based on the investigation of the arrested accused. "I was just supporting farmers", she said, breaking down.
A lawyer familiar with the developments told Scroll.in that Ravi's family had not been given any details when she was taken away - not even whether she was being arrested or detained for questioning.
While responding to queries relating to toolkit document during a press conference at the Delhi Police headquarters, Deputy Commissioner of Police (Cyber Cell) Anyesh Roy said that it is not a static document.
Numerous protesting farmers come from Punjab.
Earlier previous year, in an interview with Auto Report Africa in 2020, Ravi said, "My motivation to join climate activism came from seeing my grandparents, who are farmers, struggle with the effects of the climate crisis".
Police have not named Thunberg in the case.
The government initially tried to discredit the protesting farmers, many of whom are Sikhs, as motivated by religious nationalism.
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Arresting the activist, in her early 20s, "is an unprecedented attack on democracy", tweeted Delhi Chief Minister Arvind Kejriwal.
Ms Ravi is one of the leaders of the Indian arm of Ms Thunberg's Fridays for Future campaign movement. Overseas, the niece of U.S. Vice President Kamala Harris, Meena Harris, criticized the government on Twitter for silencing activists. "She worked closely with them to draft the Doc", the police said.
Since November, thousands of farmers, primarily from northern agrarian states Punjab and Haryana, have camped out around Delhi demanding the repeal of new farm laws that fundamentally change how crops can be sold in the market.
The arrest has also triggered global condemnation of the Delhi Police. The protests have posed a major challenge to Prime Minister Narendra Modi's Hindu nationalist-led government, which says the laws are needed to modernize Indian farming.
Police also issued arrest warrants for two other activists, Nikita Jacob and Shantanu Muluk, saying the three created the document and shared it with others.
India's foreign ministry hit out this month at "sensationalist social media hashtags and comments" by celebrities following posts from Thunberg and pop singer Rihanna, who also tweeted in support of the farmer protests.
Greta later deleted the tweet, posting an updated one.
"Sir Chhotu Ram is also credited for establishing the mandi system in India, and it is this system that the present farmers' movement seeks to protect and improve", the union body said in a statement.
A group of people gathered in Bengaluru and held placards seeking her release.
A protest was planned later in the day in the city against her arrest.