Allies of Pakistan's military ruler blocked an opposition leader, Imran Khan, from entering the country's biggest city Wednesday, two days after the government sent a former prime minister back into exile.

President Gen. Pervez Musharraf is facing a barrage of criticism for Monday's expulsion of ex-Prime Minister Nawaz Sharif, a step which removed a key political rival who could have upset his re-election plans.

In another case of authorities taking a tough line against government opponents, Khan, the leader of another opposition party, flew into Karachi on Wednesday but was halted by police at the airport and sent back to the capital, Islamabad.

Pakistani Army Kills About 40 Militant Fighters in Northwest.

The former cricket star was barred because his presence could cause unrest, city police chief Azher Farooqi said. More than a dozen Khan supporters who had been chanting anti-Musharraf slogans were detained, he said.

Khan denounced his removal as "state terrorism" by the Mutahida Qaumi Movement. The party controls the government of Sindh province, of which Karachi is the capital, and supports Musharraf at the national level.

Riot police later stopped Khan and about two dozens supporters from making a protest march to the Foreign Office where U.S. Deputy Secretary of State John Negroponte was holding talks with Pakistani officials.

"Musharraf has never had any legal authority but now he has lost all moral authority in Pakistan," Khan told reporters as his followers chanted anti-Musharraf slogans. "He's hanging in there through the help of the United States."

He alleged Washington was trying to engineer a power-sharing deal between Musharraf and another former prime minister, Benazir Bhutto, to help the U.S.-allied general extend his rule.

Khan's Tehreek-e-Insaf party, or Movement for Justice, is small. But he is an outspoken critic of Musharraf and has accused the MQM of responsibility for violence at an opposition rally in Karachi on May 12 that killed about 40 people.

Sharif, whose second term as prime minister ended with Musharraf's 1999 coup, flew into Islamabad from London. Authorities halted him at the terminal and he left a few hours later for Saudi Arabia.

Sharif had said he was ready to risk jail by coming home. But officials claimed Tuesday that when they offered him a choice between arrest on corruption charges or exile, he chose the latter.

The manner of Sharif's departure is already the subject of a complaint to the Supreme Court from his supporters.

The court ruled last month that Sharif had an "inalienable" right to return to his homeland.

It is unclear when it might rule on the latest case, but many commentators and legal experts are arguing that the government is guilty of contempt of court.

"No one is going to quite buy (the government's) argument," the Lahore-based Nation newspaper wrote in an editorial Wednesday. "For him to come to Pakistan only to be jetted to Saudi Arabia makes no sense. It is clear that he was sent against his will."

Sharif's removal could deepen Musharraf's unpopularity and reinforce impressions that he is an authoritarian leader. It could also undermine the legitimacy of legislative elections due by January.

In addition, Musharraf faces rising Islamic extremism underlined by a spate of deadly suicide attacks blamed on pro-Taliban militants based near the Afghan border.

Opposition leaders have vowed to mount street protests to unseat the government. However, demonstrations on Tuesday in several cities were small and ended without incident.

Bhutto's engagement with Musharraf restricts the scope for agitation. An agreement could defuse legal challenges to Musharraf's attempt to receive a fresh five-year term from lawmakers and quash corruption cases pending against Bhutto, who left Pakistan in 1999.

The presidential vote is due between Sept. 15 and Oct. 15 and the election commission said Tuesday that the schedule will be announced within days.