The UN Office of the High Commissioner for Human Rights has urged India to immediately release people "arbitrarily detained" in protests against misconduct by electoral authorities.
"We are concerned by reports of mass detentions in India of demonstrators, protest leaders, members of civil society organisations, lawyers and journalists, before and during today's large-scale demonstrations in Delhi," the office said in a statement on Saturday.
"All those arbitrarily detained must be immediately and unconditionally released."
Earlier, Indian police detained the head of the satirical "cockroach" online movement as well as thousands of protesters trying to take part in a major demonstration in New Delhi against the country's election commissioner.
The protest is the culmination of weeks of opposition mobilisation following allegations of misconduct during a nationwide voter-roll revision by the Election Commission of India (ECI) that purges 130 million people, mostly Muslims, from the voter roll.
Cockroach Janta Party (CJP) founder Abhijeet Dipke and two other leaders were detained before disembarking from a flight in the capital, according to a statement from the group.
"They're being denied access to their lawyers and cut off from all contact with others," it said, adding the trio had been taken to "an undisclosed location".
The CJP also said that thousands more protesters were detained in New Delhi as they attempted to march to the main Jantar Mantar protest site, defying a massive security clampdown.
"More than 15,000 protesters have been detained," Ajinkya Shinde, a CJP leader, said at a news conference in Delhi.
"We will be back; let them fill up all the jails," Shinde said.
Dozens of protesters shouted anti-government slogans as they were taken away in police vehicles, reporters witnessed.
Calls for the ECI chief Gyanesh Kumar to step down have intensified in recent weeks as opposition parties and critics said his commission's voter-roll revision could disenfranchise millions of constituents and benefit Prime Minister Narendra Modi's ruling Hindu nationalist Bharatiya Janata Party.
Ahead of the protest, authorities in the capital deployed thousands of police, closed dozens of metro stations, cancelled more than 200 trains and temporarily shut down mobile internet around the protest site.
The demonstrations are widely seen as a reprise of the "cockroach" movement's mass mobilisation in July, which led to the education minister stepping down over an examination scandal.
This time, protesters are demanding Kumar's resignation and a halt to the voter revision exercise.
The accusations are a fresh challenge for 76-year-old Modi during his third term in office.
Opposition parties have long alleged that legitimate voters are being struck off lists in India.
The issue flared up last month when the Indian Express newspaper reported that key decisions linked to the revision were likely taken without the approval of two of the three election commissioners.
The Hindu newspaper reported that more than 5,000 protesters had been detained.
Media outlets, including The Scroll and Newslaundry, said their journalists had also been detained briefly.
