The aftermath of the blasts in Delhi
Five bombs have ripped through busy shopping areas of India's capital, Delhi, within minutes of each other, killing at least 20 people, police say.
The explosions, which also injured about 90 people, are not thought to have been very powerful but happened in crowded areas.
Four unexploded bombs were also found and defused, police said.
More than 400 people have died since October 2005 in bomb attacks on Indian cities such as Ahmedabad and Bangalore.
India has blamed Islamist militant groups for these previous bombings.
CNN-IBN, a local TV news channel, said it had received an e-mail before the blasts from a group calling itself the "Indian Mujahideen".
"Do whatever you can. Stop us if you can," the e-mail reportedly said.
The same group has claimed responsibility for two other recent bombing attacks.
The Indian government has put the security agencies on high alert.
Pakistan has joined in official Indian condemnation of the attacks.
Two bombs are believed to have been planted in dustbins metres away from each other in the central shopping district of Connaught Place.
BOMB ATTACKS IN INDIA IN 2008
13 September: Five bomb blasts kill 18 in Delhi
26 July: At least 22 small bombs kill 49 in Ahmedabad
25 July: Seven bombs go off in Bangalore killing two people
13 May: Seven bomb hit markets and crowded streets in Jaipur killing 63
Police believe that at least three other devices were planted at busy markets in the Karol Bagh area, on the Barakhamba Road and in the Greater Kailash area.
Chanchal Kumar helped carry several casualties to ambulances after witnessing one of the explosions, outside a metro station.
"Around 1830 we heard a very loud noise, then we saw people running all over the place," Reuters news agency quoted him as saying.
"There were about 100-200 people around this place."
Gulab Singh, an underground train guard, saw an explosion in Greater Kailash.
"I was stepping out for a cup of tea when everything turned black in front of me," he was quoted as saying by AFP news agency. "Then everyone started running."
Television pictures show scenes of chaos at the blast scenes. Crowds milled around mangled vehicles, with debris and blood scattered across the streets.
'Enemies of humanity'
The Mayor of Delhi, Arti Mehra, said the city would not be intimidated by the "cowardly" attacks.
"They want to break the spirit of Delhi," he told reporters.
"They have tried this in other places before and they have not succeeded and they will not succeed here. They will not scare us."
Pakistan's new President, Asif Ali Zardari, "strongly condemned" the bomb attacks, expressing "shock and grief over the loss of precious human lives".
After bombings in Jaipur and Bangalore, a group calling itself the Indian Mujahideen also claimed responsibility.
While it is too early to say exactly what caused Saturday's blasts in Delhi, they appear similar to the earlier attacks.
The earlier attacks involved multiple small devices hidden in small boxes or bags and aimed at soft targets such as crowded markets, analysts say.
The devices contained shrapnel such as nuts, bolt and ball bearings while the explosives used were improvised. Islamic militants in Kashmir have tended to use military-grade explosives.