India's home minister says authorities believe the back-to-back blasts that killed at least 20 people in Mumbai were a “coordinated attack by terrorists.”

P. Chidambaram appealed for calm, after three separate explosions struck India's financial capital Wednesday evening. Authorities say the blasts wounded more than 80 people and that they expect the number of casualties to rise.

Police say two of the blasts occurred in the south of the city, near the popular Opera House and famed jewelry market Zaveri Bazaar. The other blast hit the crowded Dadar neighborhood in the center of the city.

Mumbai's police commissioner, Arup Patnaik, told reporters that bomb disposal squads are investigating. He said a preliminary survey found that an improvised explosive device was placed in an umbrella at one of the scenes. He refused to comment on whether authorities had intelligence suggesting a terrorist attack was imminent.

So far, no one has claimed responsibility for the blasts.

Mumbai is no stranger to attacks. In 2008, Pakistan-based militants laid siege to the city, killing 166 people. Domestic militant groups also are active in the country and have carried out attacks in large cities.

Pakistan's government condemned Wednesday's bombings.

U.S. President Barack Obama also issued a statement strongly condemning what he called the “outrageous attacks” in Mumbai and expressing his condolences. He pledged that the American people would stand by their close friend and partner and offer support to India's efforts to bring those responsible to justice.