Authorities say three almost-simultaneous blasts have killed 21 people and wounded more than 100 in India's financial capital of Mumbai.
All three explosions occurred in crowded parts of the city.
Indian Home Minister Palaniappan Chidambaram called the blasts a "coordinated attack by terrorists," while Maharashtra state's Chief Minister Prithviraj Chavan said it was a "challenge to Indian sovereignty."
Prime Minister Manmohan Singh appealed to the people of Mumbai to "remain calm and show a united face."
U.S. President Barack Obama condemned the "outrageous" attacks, and offered "support to India's efforts to bring the perpetrators of these terrible crimes to justice."
U.S. Secretary of State Hillary Clinton also condemned the "despicable acts of violence designed to provoke fear and division. Those who perpetrated them must know they cannot succeed."
Pakistan's president and prime minister, Asif Ali Zardari and Yousaf Raza Gilani, said in a joint statement that they were "distressed" at the loss of life.
No group claimed responsibility for the attacks.
They were the deadliest in Mumbai since November 2008 when 10 militants from the Pakistan-based Lashkar-e Taiba launched a three-day coordinated raid in which 166 people were killed.
Homegrown militant groups are also active in the country and have in the past few years carried out attacks in large cities.
compiled from agency reports