Thousands of farmers protesting at agriculture reforms have fought through police barricades and tear gas to enter Delhi's historic Red Fort complex.
They were on foot and in tractors - part of a huge rally that was planned to coincide with India's Republic Day.
Many protesters diverted from agreed routes and clashes broke out with police. One protester has died.
Mobile internet services have been suspended in parts of Delhi as security forces scramble to restore order.
The government is yet to comment on the violence, but reports say Home Minister Amit Shah held a meeting with Delhi police to discuss the situation.
Some protesters hoisted flags at the top of the dome on the Red Fort. Getty Image
The government says the reforms that spurred the protests will liberalise the agriculture sector, but farmers say they will lose income.
Tens of thousands of them have been striking on the outskirts of Delhi since November, demanding the laws be repealed. They rejected a government offer to put the laws on hold last week.
This is one of the longest farmers-led protests India has ever seen, pitting the community against Indian Prime Minister Narendra Modi and his Bharatiya Janata Party-led (BJP) government.
How did the protests turn violent?
Police agreed to allow Tuesday's rally after several rounds of talks on the condition that it would not interrupt the annual Republic Day parade, which takes place in central Delhi. They gave farmers specific routes for their rally, which would largely be confined to the outskirts.
But farmers instead converged on the iconic 16th Century fortress. They breached security and clambered onto the walls and domes of the fort, even hoisting flags alongside the national flag.
By Tuesday afternoon, police said they had removed protesting farmers from Red Fort premises.
"We came here to deliver a message to the Modi government, our job is done. We will go back now," one protesting farmer told NDTV.
While farmers at several entry points appear to have followed the agreed routes, a section of protesters at the Ghazipur, Sinnghu and Tikri borders, three of the six main entry points to the city, broke through police barricades earlier in the day.
They marched towards central Delhi, where India's parliament is located.
"Mr Modi will have to take back these black laws for sure," one protester told the BBC's Salman Ravi.
Images from the ITO metro station junction - which is on the route to central Delhi - showed police clashing with protesting farmers and using tear gas and batons against them. Protesters driving tractors appeared to be deliberately trying to run over police personnel. Local media reported injuries on both sides.
At least one protester died at the junction when his tractor overturned as police fired tear gas.
Police used tear gas in some places to try to disperse protesters. Photo: PTI
Thousands marched on foot, alongside tractors, in the rally
BBC correspondents say protesters outnumber the police at the ITO junction, and the latter are struggling to control the crowd.
"We have been appealing to farmers to go by pre-approved route but some of them broke police barricades, attacked police personnel," a senior police officer told ANI news agency. "We are appealing to farmers' unions to help maintain peace."
Farm union leaders issued similar appeals, condemning and distancing themselves from the violence.
But speaking to the BBC, Avik Saha of the All India Farmers' Struggle Coordination Committee said the authorities were to blame for the trouble.
"The violence that took place was primarily aimed at the farmers to stop them from marching in Delhi," he said. "For some reason the government feels cannot hold a parade in Delhi."
Farmers broke through police barricades to enter the city. Photo: PTI
Was the Republic Day parade affected?
The chaos ensued shortly after India's Republic Day parade.
The annual parade involves armed forces showcasing their latest equipment and floats from several states presenting their culture on a national stage. The parade is shorter and more muted this year due to the pandemic.
Mr Modi and cabinet ministers watched the official parade on Tuesday morning, but did not encounter any protesters. They were driven back to their residences before the farmers reached central Delhi.
Farmers say they want the government to roll back controversial farm reforms. Getty Image
What exactly do the laws propose?
Taken together, the laws loosen rules around the sale, pricing and storage of farm produce.
These rules have protected India's farmers from the free market for decades.
Farmers fear that the new laws will threaten decades-old concessions - such as assured prices - and weaken their bargaining power, leaving them vulnerable to exploitation by private companies.
While Mr Modi has defended them, the laws have been likened to a "death warrant" by farmer groups.
Are these reforms necessary?
Most economists and experts agree that Indian agriculture desperately needs reform. But critics of the government say it failed to consult farmers before passing the laws.
Experts also point out that the reforms fail to take into account that agriculture still remains a mainstay in the Indian economy.
More than half of Indians work on farms and the government provides farmers with generous subsidies, exempts them from income tax and crop insurance, guarantees a minimum price for 23 crops and regularly waives off debts.
"Now the government is saying, we will get out of the way, and asking us to deal directly with big businesses. But we didn't demand this in the first place! So why are they doing this to us?" Rakesh Vyas, a farmer, told the BBC recently.
Source: BBC