On foot and in tractors, the protesters were part of a huge rally planned for India's Republic Day.
Many protesters diverted from agreed routes and clashes broke out with police.
One protester died and more than 80 police officers were injured.
Mobile internet services were suspended in parts of Delhi and some metro stations closed as security forces scrambled 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.
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Tens of thousands of them have been striking on the outskirts of Delhi since November, demanding the laws be repealed. Last week they rejected a government offer to put the laws on hold.
It 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.