New Delhi is reeling from days of violence, described as some of the worst the Indian capital has suffered in decades.

Dozens of people have been killed and more than 200 were wounded since the unrest began on Sunday in largely Muslim-populated areas in northeast Delhi.

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Protests against a new citizenship law, which critics describe as divisive, discriminatory and running counter to the country's secular constitution, erupted in December and were joined by people from all religions and minorities across India.

The Hindu nationalist government of Prime Minister Narendra Modi has been criticised for failing to quell this week's violence, with opposition parties calling for the resignation of those responsible for security lapses.

Here is what you need to know:

What started the violence?

The unrest was triggered after weeks-long peaceful sit-ins in New Delhi against the new citizenship law, which opponents say discriminates against Muslims, were attacked by Hindu-nationalist mobs.

Parts of the capital descended into violence on Sunday, after a politician of the ruling Bharatiya Janata Party (BJP) leader warned Muslims against continuing with sit-ins or risk facing the BJP supporters' wrath.

Over the next four days, northeast Delhi's Karawal Nagar, Seelampur, Maujpur, Bhajanpura, Vijay Park, Jafrabad, Chandbagh, Mustafabad and Yamuna Vihar witnessed pitched battles between Hindus and Muslims.

How deadly has it been?

The violence has so far claimed the lives of at least 37 people and left more than 200 people injured. Both Hindus and Muslims have been victims, with many of the dead said to have died of bullet wounds.

In addition to street battles, there was also immense destruction of public and private property over the course of four days, with homes, shops and mosques set ablaze.

Journalists have posted on social media incidents where mobs demanded they reveal their religion, including one who was almost forced to drop their trousers - a method used to identify Muslim men in previous episodes of communal violence in the Hindu-majority nation.

Prior to the latest outbreak of violence in Delhi, local media reports recorded at least 30 deaths related to the months-long anti-CAA protests, mostly in Uttar Pradesh, a northern state home to a large Muslim population.

The police have faced accusations of looking the other way as Hindu mobs went on a rampage, killing people and damaging properties, including mosques. The police and the government have denied the allegations.

How did the authorities react?

In neighbourhoods across northeast Delhi, police imposed a British colonial-era law on Tuesday, called Section 144, which bans the assembly of more than four people statewide.

As the death toll rose, Sonia Gandhi, interim president of the main opposition party, Congress, criticised the federal and local government for being "mute spectators" to the violence.

Gandhi called for the immediate resignation of Home Minister Amit Shah, an ardent supporter of the citizenship law.

Meanwhile, Chief Minister of Delhi Arvind Kejriwal demanded that the federal government call in the army after describing the situation as "alarming".

After three days of silence, Modi called for calm and "peace and harmony" to be restored.

"I appeal to my sisters and brothers of Delhi to maintain peace and brotherhood at all times. It is important that there is calm and normalcy is restored at the earliest," he wrote on Twitter.

What is the CAA law?

Passed in December, India's Citizenship Amendment Act (CAA) eases the path for non-Muslims from neighbouring Muslim-majority nations to gain citizenship.

It aims to grant citizenship to "persecuted" Hindus, Parsis, Sikhs, Buddhists, Jains and Christians - and not Muslims - who arrived in India before December 31, 2014, from Bangladesh, Pakistan or Afghanistan.

The law is seen by opponents as discriminating against Muslims and has deepened concerns that Modi's administration is undermining India's secular traditions and imposing its Hindu nationalist agenda on the nation.

Since winning a second term last year, Modi's government has revoked the partial autonomy of Kashmir, India's only Muslim-majority state, and said it wants to conduct a nationwide citizens' register to weed out "infiltrators".

The BJP has denied having any bias against India's 180 million-plus Muslims.