MUMBAI - Twenty one people were killed and over 100 were injured in three terror blasts which rocked downtown Mumbai within minutes of each other Wednesday evening, the Press Trust of India (PTI) quoted Minister of Home Affairs as reporting.

Meanwhile, Indian NDTV reported earlier the Home Ministry has confirmed it is terrorist attack and Mumbai is on high alert. NDTV also quoted Mumbai Police sources as reporting the Indian Mujahideen (IM) is suspected to be behind the attack.

The Indian Mujahideen first came under spotlight in February 2005 when it allegedly carried out a blast in the holy city of Varanasi, killing eight people.

The group is also suspected to have carried out a series of bombing attacks in Mumbai and other Indian cities like New Delhi and Bangalore.

It is also suspected to be the mastermind of an attack upon a German bakery in the western city of Pune in February 2010, which killed more than ten people including foreigners.

Police sources said an improvised explosive device (IED) was used to carry out at least one of the three blasts.

It was the first major terror attack after the 2008 Mumbai attacks in which 160 people were killed.

The first explosion took place in south Mumbai's Zaveri Bazaar, near the famous Mumbadevi temple, in which some people were injured, said Mumbai Police spokesperson Nisar Tamboli. The bustling market also has a number of jewelry shops.

The second explosion was reported in a taxi in Dadar area in central Mumbai. The third blast was reported from south Mumbai's Opera House in Charni Road after 19:00 p.m. local time.

All three places are busy commercial and residential areas, and were bustling with people and traffic during the evening peak hours when the explosions took place.

Indian Prime Minister Manmohan Singh condemned the blasts Wednesday. "I strongly condemn the bomb blasts in Mumbai this evening," Singh said in a statement. "I appeal to the people of Mumbai to remain calm and show a united face."

Pakistani President Asif Ali Zardari and Prime Minister Yusuf Raza Gilani condemned the blasts.

"President Zardari, Prime Minister Gilani, the government and the people of Pakistan, have all condemned the blasts in Mumbai and expressed distress over the loss of lives and injuries," said the Foreign Ministry.

"The President and the Prime Minister have expressed their deepest sympathies to the Indian leadership over the loss of lives, injuries and damage to property in Mumbai," said the Foreign Ministry in a statement.

No group has claimed the responsibility for the attack.

U.S. President Barack Obama also condemned the attacks.

"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.

Obama said the U.S. government "continues to monitor the situation, including the safety and security of our citizens."