Protesters greeted Mr Blair at the Welsh Assembly
The prime minister was speaking as part of his effort to head off the growing minority in the UK who oppose the war on terrorism.
He said it was important not to flinch or falter and suggested it was "decadent" and indicative of a lack of "moral fibre" not to be able to take the terrorists on.
"I think the prime minister has been less than honest when he is trying to make this some kind of a moral crusade."
Dr Siddiqui insisted the hidden agenda behind the war was future American oil and gas interests in Central Asia.
He said the West wanted a "broad-based regime" in Afghanistan to offer security to a pipeline connecting the Caspian Sea to Pakistan.
Peaceful solution
"He knows better than anybody else this war is nothing to do with terrorism. It is all to do with controlling resources and markets."
He said dissent about the war was no more decadent than opposition to US-influenced coups against governments in Chile in the 1970s and the Congo in the 1960s.
Dr Syed Aziz Pasha, general secretary of the Union of Muslim Organisations, called on Mr Blair to look for a peaceful solution and apply the negotiating skills he used to find peace in Northern Ireland.
He told BBC News 24: "This matter cannot be resolved by bombing alone. Peaceful solutions always last longer.
"There has been no trial by an international court - you cannot become the prosecutor, jury and judge.
Patience plea
"This 'flood of evidence' if it had been presented to Mullah Omar, perhaps he would have handed over Bin Laden."
But he called for Muslims in the UK to "be patient" and not heed calls to fight in Afghanistan.
"We are advising our people to be patient because we are a peace-loving people.
"We have to try to persuade our government to stop the bombing immediately."