A powerful time bomb planted by Sikh extremists exploded Friday inside an overcrowded bus in northern Haryana state, killing at least seven people and injuring 28 others, police said.

Police in the Haryana capital of Chandigarh said officers found and defused a second bomb planted outside a theater in the city by Sikh extremists waging a bloody campaign to create the independent theocratic nation of Khalistan in neighboring Punjab state.The incidents came amid reports that Sikh extremists were planning attacks to disrupt the celebration of the 41st anniversary of India's independence from Britain, which falls on Monday.

Police said a time bomb planted in the rear of a state-owned Haryana Roadways bus exploded at 5:50 p.m. as the over-crowded vehicle approached a police barricade at Panchkula, 20 miles southeast of Chandigarh.