President of Pakistan Muslim League (PML-N) Shahbaz Sharif said on Monday that a government crackdown on his supporters made his flight back to Pakistan on Monday urgent.
Police say they have detained hundreds of PML-N activists ahead of his return from exile, but party officials put the number at over 1,200.
"In the face of these heavy-handed tactics of General (President Pervez) Musharraf, I will definitely depart London this evening for Pakistan," said Shahbaz.
"I call on the regime to realise the gravity of the situation and not to act in a manner that would bring our great country into disrepute."
Shahbaz, who faces charges in a murder case, said he was returning to clear his name and would reach Lahore late on Tuesday.
"I am ending my three years and five months of forced exile," he said during a hastily arranged news conference in the basement of a London restaurant, after a televised speech to supporters in Pakistan was pulled late on Sunday.
"This might be my last opportunity to address the people of Pakistan," he said. "If they (authorities) wish to incarcerate me, that is up to them; if they wish to try me, that is up to them; if they wish to deport me, that is up to them."
"I am returning home to contribute to the struggle and sacrifice of the party," he said. "I have a mission to accomplish and no more time to waste."
He said a televised speech had been cancelled after security agents threatened violence against the family of the station's owner.
"This was nothing less than an attack on press freedom in Pakistan by an insecure usurper for whom a simple interview by a head of a political party seemed a threat to his illegitimate rule," he said.