A powerful time bomb exploded outside a hospital in the Indian capital of New Delhi Friday, injuring four people, police said.

The blast prompted police to go on alert around the Indian capital and increase patrols. Roadblocks were set up at major intersections.Police said the time bomb exploded about at 9 p.m. outside the St. Stephen Hospital in northern New Delhi, injuring four people. One of the injured was in serious condition.

Police gave no indication as to who may have staged the attack, but Sikh extremists during the past three years have exploded bombs in New Delhi and across the northern Punjab and Haryana states in their campaign to establish an independent Sikh theocratic state in Punjab.

More than 2,400 people were killed last year in the extremist-related violence.