About 25,000 Sikhs from across the UK attended a rally in Trafalgar Square to mark an attack on their holiest shrine in India, organisers have said.
Earlier Sikhs marched from Hyde Park to Trafalgar Square to commemorate the 1984 attack on the Golden Temple in Amritsar, Punjab, north India.
Indian armed forces stormed the holiest Sikh shrine 27 years ago to flush out militants from the temple premises.
Sikh leaders called for the right to self-determination at the rally.
The Sikh Federation UK, which organised the event, said people travelled to London in coaches to attend the Freedom Rally at Trafalgar Square.
Delegations also came from Europe for the event.
Liberal Democrat MP Simon Hughes spoke at the rally alongside leaders and prominent personalities from the Sikh community in the UK, Europe and India.
The 1984 attack on the temple complex - Operation Bluestar - had been personally approved by Indian Prime Minister Indira Gandhi, to flush out militants who wanted an independent homeland of Khalistan.
There has never been agreement on the numbers killed, but Sikhs say thousands were massacred by Indian troops, many of them innocent bystanders.
Mrs Gandhi was assassinated four months later by her Sikh bodyguards. Following her death anti-Sikh rioting erupted in India which left about 3,000 Sikhs dead.
Events are also being held in Vancouver in Canada and in California to mark the 27th anniversary of the attack.