There were standoffs at police barricades in half a dozen towns in populous Uttar Pradesh state which has long been a tinderbox of communal tensions between majority Hindus and minority Muslims.

Chants of "Remove Modi" filled the air at Delhi's historic Jama Masjid mosque as protesters marched towards the centre of the capital where authorities shut some train stations and suspended the internet on Thursday to stop people gathering.

The backlash against the law pushed through parliament by the Hindu nationalist government of Prime Minister Narendra Modi on Dec. 11 marks the strongest show of dissent since he was first elected in 2014.

Police and paramilitary were deployed outside the mosque where more than 1,000 people joined the rally, Reuters witnesses said. Several carried India's flag and copies of the constitution, which states that India is a secular republic.

"We will fight till this law is rolled back. We will not back down," said Shamim Qureishi, 42, chanting outside the mosque.

The law makes it easier for people from non-Muslim minorities in Afghanistan, Bangladesh and Pakistan who settled in India prior to 2015 to obtain Indian citizenship.

Critics say the exclusion of Muslims is discriminatory and that the award of citizenship based on religion undermines the constitution.

Crowds pelted stones at police in Ferozabad, Muzzafarnagar and Ghaziabad among other cities in Uttar Pradesh and police fired tear gas in the industrial city of Kanpur to disperse the crowd.

"The situation became violent in some parts of the district, minor force was used to disperse the mob," said state police chief OP Singh.

INTERNET DOWN

Authorities shut down the internet in parts of the state to prevent inflammatory material being circulated. News channel NDTV broadcast images of a torched police van and officers chasing protesters throwing stones and wielding sticks.

From college students and academics to artists and opposition party workers, thousands of people have protested against a law that many believe is part of an unspoken agenda to divide India along communal lines.

Muslims make up 14% of India's population.

On the street below the sprawling steps of Jama Masjid, Muslims were joined by people of other faiths, including a large number of Dalits, the community that ranks at the bottom of the Hindu caste hierarchy.

Several thousand gathered on the main boulevard cutting through Jamia Milia Islamia University in south Delhi that has been the site of violent clashes with police in the last two weeks, but the protest on Friday was peaceful.

Groups of women and children, protected by human chains of young men, stood clapping and shouting slogans.

"We want justice," some of them said, condemning police action against Jamia students and the citizenship law.

"I have never seen anything like this here. People have been forced to come out on to the streets," said 55-year old Mahroof Ahmed Khan, a resident, as he walked through the demonstration.

"Hindus, Muslims, everyone is against this law. This is the power of unity," he said.

Aside from targeting Modi, protesters also vented their fury at Amit Shah, the interior minister who tabled the bill.

In Uttar Pradesh, police arrested more than 100 people, largely from the Muslim-majority areas where protests were held on Thursday.

Prominent human rights lawyer Mohammed Shoaib was detained, and another human rights activist and ex-policeman, SR Darapuri, was placed under house arrest.