David Cameron has laid a wreath at the memorial to the 1919 Amritsar massacre in India, bowing his head and standing in silence to pay respect to those who died.
Writing in the memorial book of condolence, Mr Cameron described the massacre as "a deeply shameful event in British history", adding "we must never forget what happened here".
He is the first serving Prime Minister to visit the Sikh holy city in the north-western state of Punjab, the scene of the most notorious atrocity in Britain's imperial history in India.
Troops under the command of British Brigadier-General Reginald Dyer shot into a crowd of thousands of protesters, including women, children and elderly people, until their ammunition ran out. An inquiry commissioned by the Raj colonial authorities found that 379 people were killed in the public gardens of Jallianwala Bagh, though this figure has been widely challenged by Indian sources, who put the death toll at 1,000 or more.
The atrocity helped fuel Mahatma Gandhi's struggle for Indian independence and is seen by historians as a crucial turning point on the road to the end of British rule in India.
Mr Cameron's wrote: "This was a deeply shameful event in British history, one that Winston Churchill rightly described at the time as 'monstrous'. We must never forget what happened here, and in remembering we must ensure that the United Kingdom stands up for the right of peaceful protest around the world."
Mr Cameron made his entry in the book of condolence seated at a table before a memorial plaque which read: "This place is saturated with the blood of those Indian patriots who were martyred in a non-violent struggle to free India from British domination."
Mr Cameron explained why he decided not to apologise for the atrocity, saying: "In my view, we are dealing with something here that happened a good 40 years before I was born, and we are dealing with something that... the British Government rightly condemned at the time," he said.
"I don't think the right thing is to reach back into history and to seek out things that we should apologise for. I think the right thing to do is to acknowledge what happened, to recall what happened, to show respect and understanding."
Before visiting the massacre site, Mr Cameron toured Amritsar's Golden Temple, the holiest site in the Sikh religion. Barefoot and wearing a blue bandana head-covering, the Prime Minister visited the kitchens which feed thousands of pilgrims every day and tried his hand at flipping chapatis before entering the historic shrine itself.