"If that stage comes, and when it comes, I assure you that our parliament, our democracy, shall take the action properly deemed in our constitution and in our law," he said.
He said that Western intelligence agencies had not offered firm evidence to justify claims that the attacks were orchestrated from Pakistani soil and that he would not jump to conclusions until a full investigation had been conducted.
Mr Zardari said claims that the sole surviving attacker had been identified by his own father as coming from Pakistan had not been proven. The man has been named as Mohammed Ajmal Amir Qasab and is in Indian police custody.
The Pakistani president also said Hafiz Saeed, the leader of the Jamaat-ud-Dawa charity, would remain under house arrest.
Pakistan says that terrorism is a common enemy
"Let me assure you that if there is any investigation to be found pointing towards his involvement in any form of terrorism, he shall be tried for that reason," the president said.
Jamaat-ud-Dawa is accused of being a front for Lashkar-e-Taiba, a charge it denies.
Mr Zardari said that he had asked India to co-operate in an investigation and he would not leap to judgement while that investigation was continuing.
He said that while he was not in denial about Lashkar-e-Taiba's continued activities, ''when you ban an organisation they emerge in some other form''.
Our correspondent says the president's denial will further dismay India, and frustrate Britain and America, which are exerting intense pressure on Pakistan to take action.
Meanwhile, Pakistani Foreign Minister Shah Mehmood Qureshi has expressed regret after his Indian counterpart said peace moves between the countries were on hold because of the Mumbai attacks.
But Mr Qureshi said relations between the two countries - which both have nuclear arms - would recover.
"This unfortunate incident has been a setback... to the extent that our composite dialogue is at a pause at the moment," Mr Qureshi said.
"But I am confident that we will overcome this hiccup... the future of Pakistan demands good neighbourly relations with India, so it's my responsibility to develop good bilateral relations."
Mr Qureshi said that the two countries had a "common objective, a common challenge and a common enemy" in relation to "terrorism".
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.