to condemn the hanging, a Reuters witness said.
Five militants stormed the heavily guarded parliament complex in New Delhi on Dec. 13, 2001, armed with grenades, guns and explosives, but security forces killed them before they could enter the main chamber. Ten other people, most of them security officers, were killed in the clash.
Guru, an Indian national, was convicted for helping organise arms for the attackers and a place for them to stay. He had denied any involvement in the conspiracy.
India said the Pakistan-based Jaish-e-Mohammad militant group was responsible. The group fights Indian rule in Muslim-majority Kashmir.
The hanging last year of Mohammad Ajmal Kasab, the lone surviving Pakistani militant involved in the 2008 Mumbai attacks, after a long lull in executions, prompted speculation that India would move quickly to execute Guru.
But unlike Kasab's execution, which sparked celebrations in the streets, Guru's case was seen as more divisive.
Some Kashmiri leaders warned that hanging Afzal would fuel the revolt in India's part of the Himalayan region in which tens of thousands of people have been killed since 1989.
Curfews were imposed in Srinagar, the region's summer capital in the Kashmir valley, and major towns including Baramulla, Guru's home town.
Kashmir is divided between India and Pakistan, both of which claim the region in full and rule it in part. They have fought two of their three wars over the region.
India has long accused Muslim Pakistan of arming and funding militants to fight Indian forces in Kashmir. Pakistan says it only provides moral support to the fellow-Muslim people of Kashmir, who Pakistan says, face heavy-handed Indian rule.
The dispute, a legacy of the division of the sub-continent at the end of British rule, is the main factor souring relations between the neighbours.