Officials already started visiting the scene of a Mumbai blast today where 21 people were killed and more than 100 were injured.

About five hours following the Mumbai explosions today, Union home minister P Chidambaram visited the scene to get an assessment of the scene of the triple blasts, the Times of India is reporting.

Chidambaram, who is said to have had a grim look, was accompanied by some high-ranking home department and police officials, the paper reports.

The three separate bombs went off in crowded districts of Mumbai during rush hour.

Reuters reports that the blasts were the biggest militant attack on India's financial capital since 2008 assaults blamed on Pakistan-based militants.

No one has come forward claiming responsibility for the blasts.

The Hindustan Times is reporting that two of the blasts were planted in a motorcycle, and the third was concealed under an umbrella placed by the roadside.

Mumbai Police also told the Hindustan Times that the low-intensity explosions went off at Zaveri Bazaar, Opera House and Kabutar Khana in Dadar, which are all densely-crowded places during the evening rush hour.

Twelve people were killed at Zaveri Bazaar, and nine at Opera House, but no deaths in Dadar though there were eight reported injuries, the article states.

Chidambaram said the blasts a coordinated attack by terrorists, while Chief Minister Prithviraj Chavan said it was another attack on the heart of India, the paper reports.

Mumbai police commissioner Arup Patnaik told Hindustan Times that they didn't know whom to suspect.