Three consecutive bomb blasts tore through India's financial capital, Mumbai, Wednesday, killing at least 21 people and wounding more than 100 others.

The explosions occurred within 20 minutes of each other in crowded areas of the city during the evening rush-hour — two in the south and one in central Mumbai.

No one has claimed responsibility, but Indian Home Minister P. Chidambaram said the government believes the blasts were a “coordinated terror attack” because of their close timing. He urged the people of Mumbai and those across the country to remain calm and maintain peace. Chidambaram is due to visit Mumbai Thursday.

The home ministry also has ordered security heightened across the country.

Police believe the blasts were caused by improvised explosive devices. They say that in two of the blasts, the bombs were attached to motor vehicles, while the third occurred on top of an electrical metering box above a billboard.

Indian Prime Minister Manmohan Singh condemned the attacks, as did the Pakistani government.

U.S. President Barack Obama issued a statement strongly condemning what he called the “outrageous attacks” in Mumbai and pledging “support to India's efforts to bring those responsible to justice.”

Wednesday's attack is the worst to hit Mumbai since Pakistan-based militants laid siege to the city in 2008, killing 166 people.

It happened just two days after the fifth anniversary of a series of train bombings in Mumbai that killed more than 180 people. Indian authorities blamed that attack on Pakistani militants.