Thousands of police and paramilitary personnel were deployed across the city on Saturday, mobile internet services were suspended in the central business district and dozens of metro stations and markets were shut as authorities moved to stop a demonstration organised by the Cockroach movement.
The protest was aimed at Chief Election Commissioner Gyanesh Kumar over a revision of voter lists that critics say disproportionately benefits Prime Minister Narendra Modi's ruling Bharatiya Janata Party (BJP).
The movement had planned to gather thousands of people at Jantar Mantar in central Delhi, but the area was sealed off by security forces.
A home ministry official, speaking on condition of anonymity, told Reuters more than 2000 people were detained in Delhi. Local media said about 400 protesters were held in India's financial hub, Mumbai.
The United Nations human rights office expressed concerns over mass detentions before and during the protests.
"We call on the authorities to ensure that the right to peaceful assembly is fully respected and fulfilled," UN Human Rights Spokesperson Thameen Al-Kheetan said in a statement.
He urged authorities to engage in "meaningful dialogue" to address the "broadly held" concerns expressed.
The Indian mission to the UN in Geneva, however, called the UN's concerns "misplaced" and said they constitute "an unwarranted interference in the internal affairs of India".
It stressed that the right to peaceful assembly was guaranteed by India's Constitution.
"As in every democracy, Indian citizens exercise this right subject to reasonable restrictions in the interest of public order," the statement added.
Police detained Cockroach Janta Party founder Abhijeet Dipke and senior leaders Saurav Das and Ashutosh Ranka when their flight from Mumbai landed in Delhi, the group said in a statement.
Television footage showed Dipke challenging officers who approached him on board the aircraft.
"I have come to be arrested, arrest me ... I am not afraid of you guys," he said.
"No, this is not an arrest," one officer replied. "Please co-operate."
The movement condemned what it called the "authoritarian detention" of its leaders, demanded their release and vowed to continue protesting.
"We will continue our protest tomorrow, day after ... until Gyanesh Kumar resigns. We will be on the streets," CJP leader Ajinkya Shinde told reporters.
CJP also said authorities refused to provide updates on the leaders' whereabouts or health, even after news arrived of the death of Dipke's grandmother. Police said Dipke was informed of her death and was offered help to return home for the funeral.
Across the capital of about 20 million people, security forces set up checkpoints on major roads and entry points, boarded buses searching for protesters and inspected vehicles.
Barricades and concrete blocks disrupted traffic while anti-riot vehicles equipped with tear-gas launchers guarded key routes.
The demonstrations represent one of the most serious tests yet for Modi, stemming from allegations that election authorities improperly revised voter rolls ahead of recent state polls.
Protests intensified after the Indian Express reported last week that two election commissioners had questioned Kumar's voter-list revisions that removed roughly 130 million names from voter lists.
The unrest also comes ahead of a series of important state elections beginning in early 2027.
