By Mark Trevelyan, Security Correspondent

LONDON, Oct 19 (Reuters) - Benazir Bhutto's return to Pakistan was always going to be fraught with risk. In the event, it took just hours for the nightmares of her Pakistani supporters and U.S. backers to materialise.

Though the former prime minister herself escaped unharmed, more than 100 people were killed and twice as many wounded as a suspected suicide bomber struck her homecoming parade through Karachi streets thronging with vast crowds.

Washington hopes Bhutto's return after eight years of self-imposed exile can help stabilise the world's only nuclear-armed Islamic state and shore up President Pervez Musharraf as he faces a rising al Qaeda and Taliban tide.

But the worst-case scenario, highlighted by Friday's carnage, is that a prospective power-sharing deal between the two leaders may only serve to energise the militants, incensed by her backing for the general who took power in a 1999 coup.

"Even before Benazir got to Pakistan, we had said this is a very ill-thought-out deal between Musharraf and Benazir Bhutto," said M.J. Gohel, head of the Asia-Pacific Foundation think-tank in London.

"We said that the people who've supported this deal in the West have not fully understood the internal dynamics of that country, and someone is going to get killed."

Bashir Riaz, a Bhutto spokesman in London, told Reuters: "This is very, very disturbing ... There was a danger, but we didn't know it would materialise on such an extensive scale."

He said Bhutto was likely to hold meetings with leaders of her Pakistan People's Party to decide her next move, but he expected her to press on with her campaign for parliamentary elections.

"She is there to lead the party in the general election. I think these things will not deter her. She is a determined person and a very courageous lady," Riaz said.

He said it was "not proper" to speculate on who was behind the attack: "There are so many enemies, political and non-political. One cannot point the finger at this stage."

One Pakistani official had told Reuters on the eve of Bhutto's return there were intelligence reports that three different groups linked to al Qaeda and the Taliban were planning suicide attacks against her.

Analyst Gohel said other groups could also be in the frame. "Karachi is a hotbed of sectarian killings and other extremist radical elements," he said.

Bhutto announced the date and place of her return from exile weeks ago, giving her enemies ample time to prepare one of the worst blasts in the country's history, despite the presence of some 20,000 security personnel at the procession.

At a meeting of her party in London two weeks ago, the two-times prime minister had spoken to reporters at length about rising militancy in Pakistan and said she feared there could be violence in the streets.

But in a column published in The Times on the day of her return, she declared: "I will not be intimidated ... Despite threats of death, I will not acquiesce to tyranny, but rather lead the fight against it."

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