India on Saturday marked the 100th anniversary of the Amritsar massacre, one of the worst atrocities of British colonial rule for which London has yet to apologize.

The Jallianwala Bagh massacre, as it is known in India, saw British troops fire on thousands of unarmed people in Amritsar, on April 13, 1919.

The number of casualties is unclear, with colonial-era records showing about 400 deaths, while Indian figures put the number at closer to 1,000.

Former British prime minister David Cameron described it as "deeply shameful" in a visit to the northern Indian city in 2013 but stopped short of an apology.

In 1997, Queen Elizabeth II laid a wreath at the site but her husband Prince Philip stole the headlines by reportedly saying that Indian estimates for the death count were "vastly exaggerated."

This week, British Prime Minister Theresa May told the House of Commons that the massacre was "a shameful scar on British Indian history." She said, "We deeply regret what happened and the suffering caused," but she, too, avoided saying she was sorry.

Amarinder Singh, the chief minister of Punjab state, said May's words were not enough.

He said "an unequivocal official apology" is needed for the "monumental barbarity." 

Singh said thousands attended a candlelight march on Friday in memory of the victims.

Around 10,000 unarmed men, women and children had gathered in the Jallianwala Bagh walled public garden in Amritsar on the afternoon of April 13, 1919.

Many were angry about the recent extension of repressive measures and the arrest of two local leaders that had sparked violent protests three days before.

The day was also a big spring festival, and the crowd - estimated by some at 20,000 - included pilgrims visiting the nearby Golden Temple sacred to Sikhs.

Brigadier-General Reginald Edward Harry Dyer arrived with dozens of troops, sealed off the exit and without warning ordered the soldiers to open fire.

Many tried to escape by scaling the high walls surrounding the area. Others jumped into an open well at the site as the troops fired.

One of several eyewitness accounts compiled by two historians and published in the Indian Express newspaper this week described the horror.

"Heaps of dead bodies lay there, some on their backs and some with their faces upturned. A number of them were poor innocent children. I shall never forget the sight," said Ratan Devi, whose husband was killed.

"I was all alone the whole night in that solitary jungle. Nothing but the barking of dogs, or the braying of donkeys was audible. Amidst hundreds of corpses, I passed my night, crying and watching," she said.

Dyer, dubbed "The Butcher of Amritsar," said later it was a necessary measure, and that the firing was "not to disperse the meeting but to punish the Indians for disobedience."