NEW DELHI, Feb 13 (Reuters): India's government and opposition parties cautiously welcomed Pakistan's investigation into the Mumbai attack on Friday, in a sign diplomatic tension between the two nuclear powers could wane.

Pakistan's government said on Thursday for the first time that November's attack was launched and partly planned from Pakistan, and it was holding in custody a ringleader and five other suspects.

The acknowledgement by Pakistan of partial involvement of some of its citizens in attacks that killed 179 people came after nearly three months of angry rhetoric between the two nations, which have gone to war three times since independence in 1947.

The possible diplomatic thaw may also benefit U.S. envoy Richard Holbrooke, now traveling in the region as he seeks to help bring stability to Pakistan and Afghanistan.

"This is a positive step. We hope that the matter will be taken to its logical conclusion where the perpetrators are penalized," Foreign Minister Pranab Mukherjee told reporters.

Later in a statement before the Indian parliament, Mukherjee said Pakistan must dismantle all militant infrastructure on its soil.

"Hon. members are aware of the prevarication, denial, diversionary tactics and misplaced sense of victimhood which characterized Pakistan's reaction from early days after the Mumbai attack," Mukherjee said.

"I do not discount in any way either their intent or their sincerity, but the fact remains that the overwhelming response of official Pakistan to the Mumbai attack was not appropriate to a terrorist attack where innocents were massacred in cold blood."

A waning of tensions could benefit India's Congress party-led ruling coalition ahead of general elections due by May, but analysts say Pakistan was unlikely to make any significant gesture until a new government was installed in New Delhi.

"If Pakistan is going to make a gesture to India, they will make it with a new Indian government rather than a government with only a few months left in office," political columnist Amitabh Mattoo told Reuters.

India's Hindu nationalist opposition, the Bharatiya Janata Party (BJP), said Pakistan had emerged from denial mode.

"With this revelation, now the next stage should be eliminating the terrorist camps inside Pakistan and crushing the association between state actors and these terrorist groups," BJP spokesman Rajiv Pratap Rudy said.

Indian newspapers welcomed Islamabad's acknowledgement of Pakistani involvement in the Mumbai attack.

"Pak blinks, finally," read the headline in the Times of India newspaper, while the Mail Today led a story with "Finally, Pakistan admits some guilt."

Pakistan's move came after U.S. pressure for Islamabad to act, and could signal Pakistan is reacting to President Barack Obama's policy of tying aid closer to progress made on battling militants and bringing political stability.

In Mumbai, however, the news of Pakistan's acknowledgement of involvement by some Pakistanis in the attacks was received with a strong dose of skepticism. Many citizens said India should stand firm.

"It's a start, and hopefully the pressure from the U.S. and other countries will continue," said Aparna K, an advertising executive.

"I think we should also stay firm. For too long India has been accepting of everything, we have not pushed hard enough. It's about time we stood our ground and pushed for action," she said.