Speaking to the newspaper in London, Sharif said he will return to Islamabad within two weeks for an eve-of-Ramadan campaign against Musharraf, who took power in a bloodless coup in 1999.
Sharif has been living in exile in Saudi Arabia. Last week, Pakistan's Supreme Court lifted the ban on him returning to Pakistan.
He said in the interview support for Musharraf was sinking.
"Today the people, civil society, the judiciary, the political forces and the media are on one side, and the dictator and his shrinking support are on the other side", Sharif said.
He also warned the United States not to "equate Mr. Musharraf with Pakistan," or it would see an increase in Islamist extremism in the region.
"I do feel let down by the United States," Sharif told the Financial Times. "(But) I will not use the word betrayed."