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Mumbai, India (CNN) -- Three bomb blasts rocked India's largest city, Mumbai, in congested areas during the evening rush hour Wednesday, killing at least 21 people and injuring more than 100 others.
Prithviraj Chavan, Maharashtra state's chief minister, said it was too early to talk about suspects but at least one of the apparently coordinated blasts was "quite powerful." He warned the death toll could rise.
Indian Home Minister P. Chidambaram called the explosions a "coordinated attack by terrorists" and appealed for calm as Mumbai residents voiced anger at the government because their city has been a repeated target for terror.
"The entire city of Mumbai has been placed on high alert," said Chidambaram, who inspected the sites late Wednesday and early Thursday and saw the injured at a hospital. "I would appeal to the people of Mumbai to remain calm and maintain the peace."
The minister was expected to provide an update on the investigation midmorning Thursday.
The bombings Wednesday brought back haunting memories in a city that has suffered before, including the massive assault by Pakistani gunmen that killed 164 people in November 2008.
Forensic experts and security forces using sniffer dogs pored over the bloody scenes, looking for evidence. Dazed survivors were comforted and police hosed down burning debris in the streets.
Mumbai Police Commissioner Arup Patnaik said at one scene, an improvised-explosive device was placed in an umbrella near a motorcycle.
CNN's sister station CNN-IBN reported at least 113 people were injured.
The blasts occurred within minutes of each other in the areas of Opera House, Zaveri Bazaar and Dadar, all busy commercial hubs that were teeming with people in the evening rush hour.
"Mumbai is a prime target," Chavan said of India's financial capital. "Terrorists will obviously attack where it hurts most."
He said Mumbai was better prepared than in the past but the "element of surprise is on the side of those who attack."
The area in Dadar is near a train station used by millions of commuters. On July 11, 2006, a series of seven explosions killed at least 174 people on crowded Mumbai commuter trains and stations.
The U.N. Security Council condemned the terrorist attacks. The United States offered its "full support and assistance to India" and was monitoring the situation, the State Department said.
U.S. President Barack Obama, who visited Mumbai last year and signed a condolence book for the victims of the 2008 attack, condemned Wednesday's attack.
"I strongly condemn the outrageous attacks in Mumbai, and my thoughts and prayers are with the wounded and those who have lost loved ones," Obama said in a statement.
"The U.S. government continues to monitor the situation, including the safety and security of our citizens. During my trip to Mumbai, I saw firsthand the strength and resilience of the Indian people, and I have no doubt that India will overcome these deplorable terrorist attacks."
Secretary of State Hillary Clinton is scheduled to travel to New Delhi and Chennai next week, but her spokesman said there were no immediate plans to alter her travel plans. Clinton "will have an opportunity to re-affirm our commitment to the U.S.-India partnership when she travels there next week," Mark Toner said.
As in past attacks, the blasts Wednesday targeted congested areas. Mumbai police blamed makeshift bombs and told CNN's sister network CNN-IBN that one was left in a car; another in a motorcycle.
"The sound was absolutely deafening," said Hemant Mehta, who was in the Opera House area, near a diamond market that serves as a small epicenter of the city's economy. "People ran and panicked. Words are insufficient to describe (the scene)."
Mehta asked for top officials to protect citizens in the city.
Chavan said the blast at Opera House was the most powerful.
Zaveri Bazaar is near a Hindu temple, in which some people were injured, Mumbai police representative Nisar Tamboli told CNN-IBN. Zaveri Bazaar was one of the scenes of a twin bombing in 2003 that killed 54 people and the area also came under attack in the 1993 bombings that left 257 dead.
Wednesday's attack was the first since the 2008 terrorist siege on the city's main train station, two luxury hotels and a Jewish cultural center. Mumbaikars said the targeting of such congested areas meant that the blasts were intended to inflict high casualties.
Authorities also issued high security alerts for the Indian capital, New Delhi, and the eastern city of Kolkata, CNN-IBN reported.
Witnesses told CNN-IBN that their window panes shook and they heard the thundering boom of the blasts. Workers who had not left their offices were advised not to venture out.
CNN's Mallika Kapur in Mumbai and Pam Benson in Washington contributed to this report.