NEW DELHI, India (CNN) -- Three explosions ripped through New Delhi within minutes of each other Saturday evening, killing at least 55 people.Most of the deaths occurred at a marketplace crowded with thousands of people getting ready for India's festival of lights, the state of Delhi's chief minister, Sheila Dikshit, told CNN.She said at least 31 people died at the Sarojini Nagar marketplace in central Delhi and the others were killed at the Paharganj marketplace, both packed with people preparing for the Hindu holiday of Diwali, India's largest festival.Police reported that the blasts wounded 155 people. (Watch a report on the explosions -- 2:18 )While it is not clear what caused the blasts, Indian Prime Minister Manmohan Singh's office said he believes it was a terrorist attack."This kind of cynical attack on the people of India is just not acceptable," Singh's media adviser, Sanjay Baru, told CNN.No group has claimed responsibility.The explosions occurred at two Delhi marketplaces and in the neighborhood of Govindpuri. Police said several people were critically injured in Govindpuri, but there were no reports of deaths. (A look at where the blasts happened)"It's a very sad day for all of us because Delhi is celebrating a festive season," Dikshit said.U.S. Secretary of State Condoleezza Rice offered her condolences in a written statement Saturday night."The United States condemns the multiple terror attacks in New Delhi, India," she said. "These acts are made more heinous in that they deliberately targeted innocent civilians preparing for holiday celebrations. Our thoughts and prayers are with the families of the victims."Witness: Bag explodedA man who owns a store near the Sarojini Nagar market told CNN's Satinder Bindra that he noticed an abandoned bag sitting near his store and warned people to get away. As people scattered, the owner said the bag exploded, gutting several nearby stores and causing a fire that spread to other buildings.The explosion caused a stampede as people in the crowded market tried to get away.Authorities said bodies are still being pulled from the debris, and they fear the death toll will rise.Many women and children sustained serious injuries.The blasts triggered fears throughout the capital, shattering some of the festive atmosphere that precedes Diwali.Fireworks are often set off in advance of Diwali, so many people who heard the blasts initially thought they were firecrackers.Authorities urged people not to panic and to stay away from crowded areas.Indian authorities had issued some warnings in advance of Diwali, saying people should beware of the possibility of violence. Such warnings have become standard in recent years, and there were no warnings of specific attacks being planned.While Indian authorities are blaming terrorists for Saturday's attack, they are not saying if a particular group is believed to be responsible.Pakistan's foreign office issued a statement condemning the attack, calling it a criminal act of terrorism, and expressing the government's sympathies with the people of India.Delhi was the site in December 2001 of an attack on Parliament in which nine police officers, a government staffer and all five terrorists were killed. It was blamed on Islamic militant groups backed by Pakistan, although Hindu-Muslim tensions are normally centered in the disputed region of Kashmir.CNN's John Raedler and Ram Ramgopal contributed to this report