Indian President Abdul Kalam has led his country in a two-minute silence as tribute to the victims of the Mumbai train bombings.
People in Delhi and elsewhere lit candles
It began at 1825 local time (1255 GMT) - the time of the attacks which left at least 182 people dead, according to police figures, exactly a week ago.
Sirens sounded across Mumbai marking the memorial service.
People gathered at the site of the blasts, in railway stations on the city's Western Line.
Many lit candles in memory of the victims. Taxis and cars came to a halt while cinemas interrupted films as a mark of respect.
The authorities have said that the powerful explosive RDX was used in the train bombings.
Ammonium nitrate was also used in the attacks, a senior official said.
Fragments of timer devices have also been found, scientific reports say. No significant arrests have been made.
India has accused militants based in Pakistan of involvement in the attacks but Pakistan has rejected the suggestion.
India says the attack has made it difficult to sustain the peace process with Pakistan, but says it is still committed to it.
The head of the anti-terrorist squad, KP Raghuvanshi, said the investigation was at a "critical stage", but declined to say who might have carried out the bombings.
Earlier, police said they expected to make a breakthrough in the investigation soon.
Police have questioned hundreds of people, mostly Muslims.
Indian security officials have suggested that the Mumbai bombings bore the hallmarks of Lashkar-e-Toiba, a Kashmiri militant group operating from Pakistan.
But the Indian government has not directly accused the group, which has denied any responsibility.
A Muslim organisation banned in India, the Students' Islamic Movement (Simi), has also denied involvement in the attacks.
Indian Prime Minister Manmohan Singh said on Sunday there had to be "firm commitment" that Pakistani territory was not used to support terrorist acts directed against India.