By David Loyn
BBC News, Delhi
PM Singh came close to threatening retaliation against Pakistan
In India the opposition BJP have used the opportunity of the Mumbai violence to put pressure on the government.
Front page newspaper adverts appeared on Friday even while shooting was still going on, saying the incident shows that the Congress government is 'unwilling and incapable' of dealing with terrorism.
With the country in the middle of crucial state elections which could determine the timing of the next general election, Prime Minister Manmohan Singh is facing intense scrutiny.
Following the attacks, he has been seen visiting the injured in hospital, alongside Congress party president Sonia Gandhi.
He has already promised to strengthen anti-terrorist laws, and in a TV address came close to threatening retaliation against Pakistan if their involvement in the attacks can be proved.
"We will take up strongly with our neighbours that the use of their territory for launching attacks on us will not be tolerated, and that there would be a cost if suitable measures are not taken by them," Mr Singh said.
The Indian Navy has seized two Pakistani merchant ships and is investigating the possibility that they dropped off the militants who then came ashore in fast boats.
Pakistani FM Qureshi vowed to cooperate with the Indian investigation
They are linking this with the discovery of a trawler, found abandoned off the Indian coastline on Thursday with its captain dead.
Pakistan's denials of involvement have been clear and unambiguous.
The Pakistani ambassador to the US, Hussein Haqqani told the BBC that his country had suffered from terrorism just as much as India had, and offered every assistance in bringing the attackers to justice.
Analysts in Pakistan have been pointing instead to the possibility that these militants are home-grown Indian extremists, operating without external support.
The incident comes just as the first democratic government in Pakistan since the coup in 1999 has made overtures for better relations with India.
For the first time, President Asif Ali Zardari made the quite unexpected unilateral offer to make no first use of nuclear weapons in any conflict.
On Tuesday, home affairs ministers from the two countries met in Islamabad, and Pakistan's foreign minister Shah Mahmoud Qureshi is by chance currently visiting India.
Such contacts are opposed by significant parts of the Pakistani army and particularly its intelligence service, the ISI, who have in the past inspired terrorist attacks in India to stop just such an improvement in relations between the two countries.
Indian papers said the government had failed to protect its citizens
Feeling encircled - with India to their east allied with Afghanistan to their west - analysts believe they have taken the option of encouraging attacks by proxies, Islamists inspired to wage unconventional war.
An armed assault by militants on the Indian parliament in 2001 led to a significant worsening in relations that escalated into troops on both sides being sent to confront each other across their shared border.
A further possibility though is that this attack was carried out from Pakistan, but beyond the control either of Pakistan's democratic government or its military establishment.
The war in Afghanistan has led to a further radicalisation of politics in Pakistan's North-West Frontier Province, where a "Pakistani Taleban" have emerged.
Allied with foreign fighters from al-Qaeda, they have both the financial power and political will to carry out attacks of the sort seen in Mumbai.