A petition was filed meanwhile in India's top court challenging the lockdown.
The rigorous security measures followed India's government decision this week to revoke the special status of Jammu and Kashmir and downgrade the Himalayan region from statehood to a territory. Muslim-majority Kashmir is claimed in full by both India and Pakistan, and rebels have been fighting Indian rule in the portion it administers for decades.
State-run All India Radio, which reported on the arrests without details, also said that cross-border firing by Indian and Pakistani troops hit the Rajouri sector of the Indian-controlled Kashmir late Wednesday.
Prime Minister Narendra Modi is expected to address the nation on Thursday to discuss Kashmir. His national security adviser, Ajit Doval, visited the region on Wednesday to assess the law and order situation.
Activist Ali Mohammed told the New Delhi Television news channel that he has been organizing ambulances to carry sick poor people to hospitals in Srinagar, the main city in India's portion of Kashmir, as local residents can't even use phones to ask for medical help.
"It's hell," a patient told the television channel.
Opposition Congress party activist Tehseen Poonawalla said he expected the Supreme Court to hear his petition on Thursday seeking immediate lifting of curfew and other restrictions, including blocking of phone lines, internet and news channels in Kashmir.
He also sought the immediate release of Kashmiri leaders who have been detained, including Omar Abdullah and Mehbooba Mufti.
In response to India's action, Pakistan on Wednesday said it would downgrade its diplomatic ties with New Delhi, expel the Indian ambassador and suspend trade. Prime Minister Imran Khan told Pakistan's National Security Committee that his government will use all diplomatic channels "to expose the brutal Indian racist regime" and human rights violations in Kashmir, the government's statement said.
India regretted Pakistan's steps and said in a statement that "the intention behind these measures is obviously to present an alarming picture to the world of our bilateral ties."
The External Affairs Ministry said it was not surprising that Pakistan has negatively perceived India's decision to end Kashmir's special status as Islamabad "has used such sentiments to justify its cross-border terrorism.'"
Describing India's latest steps in Kashmir as internal affairs, the statement urged Pakistan to review its decision to downgrade ties so that normal channels for diplomatic communications are preserved.'
India and Pakistan have fought two of their three wars since independence over control of Kashmir. The first war ended in 1948 with a U.N.-brokered cease-fire that left Kashmir divided and promised its people a U.N.-sponsored referendum on the region's future.
Pakistan said it would ask the United Nations to pressure India to reverse its decision to downgrade Kashmir ffom a state to two separate territories. The region also lost its right to fly its own flag and make many of its own decisions.
The government in Islamabad also said it would give diplomatic, political and moral support to people living in Kashmir and their "right of self-determination."
Kashmir is India's only Muslim-majority state and most people there oppose Indian rule. The insurgency that began in 1989 and India's ensuing crackdown have killed more than 70,000 people.