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, the scene of the most notorious atrocity in Britain's imperial history in India when hundreds of people died after British troops opened fire on a protest.
In the condolence book, Mr Cameron 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."
An inquiry commissioned by the Raj colonial authorities found that 379 people died 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 - portrayed in Richard Attenborough's film Gandhi - was seen as an important turning point on the road to the end of British rule in India. The words of remorse were stronger than the comments of the Queen when she laid a wreath at the memorial in 1997 and described the massacre as a "distressing" example of the "moments of sadness" in Anglo-Indian history.
Mr Cameron's visit to the site comes on the final day of a three-day trip, which has seen him lead the largest trade mission ever to travel overseas with a prime minister.
Mr Cameron was shown around the Jallianwala Bagh Gardens by descendants of some of those who came under fire from troops under British command in 1919. They pointed out the wall where bullet holes can still be seen from the massacre and the Martyrs' Well where many people died after seeking shelter from the volleys of bullets.
Speaking after the visit, the memorial's secretary Sukumar Mukherjee, whose grandfather survived the shootings, was asked if Mr Cameron's words constituted an apology. He replied: "He has come here, he has paid his tribute here. It is more than an apology."
But other descendants were not so happy. Sunil Kapoor, 36, whose great-grandfather Wassoo Mal Kapoor died, said: "I'm not satisfied because he didn't meet the descendants. If you feel shameful then why not make an apology?"