KARACHI — Thousands of Benazir Bhutto supporters surged toward Karachi on the eve of the former premier’s return from exile, as she declared any Islamic militant assassin targeting her would “burn in hell.”

Meanwhile, Pakistan’s top court heard challenges to the legality of President Musharraf’s re-election as president.

Police were readying bomb disposal squads and sealing roads ahead of Ms. Bhutto’s planned return to this chaotic city of 15 million people tomorrow, where she hopes 1 million people will greet the end of her eight-year exile.

Negotiations with General Musharraf that could see the archrivals team up in an American-friendly alliance to fight Al Qaeda and the Taliban have already produced an amnesty covering the corruption cases that made her leave Pakistan in 1999. Ms. Bhutto hopes to secure a third term as prime minister after January elections.

“My return heralds for the people of Pakistan the turn of the wheel from dictatorship to democracy,” Ms. Bhutto said at a news conference in Dubai, flanked by her two daughters and her husband.

But Ms. Bhutto recently courted controversy in Pakistan by saying that she would cooperate with the American military in targeting Osama bin Laden, and authorities here warned that militants could launch suicide attacks and roadside bombings against her.

Asked about such threats, Ms. Bhutto said Islam forbids suicide bombings and attacks on her. “Muslims know if they attack a woman they will burn in hell,” she said.

The government of Sindh province, of which Karachi is the capital, appealed to Ms. Bhutto to abandon plans for a snail-paced 10-mile grand procession into Karachi, saying it would leave her vulnerable.

It said the main threat was from Taliban and Al Qaeda.

“We have informed Ms. Bhutto and her team of the situation and advised them to cut short the program instead of going for a 18 to 20 hours-long procession as this would be tantamount to inviting trouble,” Home Secretary Ghulam Muhammad Mohtarem of Sindh said.

There is sure to be a huge crowd.

With Bhutto’s Pakistan People’s Party (PPP) already mobilizing rallies and convoys of supporters expected to arrive from its strongholds across Sindh by late today, many observers believe more than 100,000 will turn out. The party is predicting there will be more than 1 million.