ISLAMABAD (Reuters) - Pakistan celebrated 60 years of independence from British rule with flag-raising ceremonies and fireworks but political troubles for a pro-U.S. president and fears of militant violence subdued festivities in several cities.

President Pervez Musharraf faces one of his toughest periods since taking power in a 1999 coup, with growing political opposition to his attempt to win a second term and rising Islamist violence that has worried his ally, the United States.

Islamist militants have stepped up attacks across Pakistan, including the capital, after a siege and bloody military assault last month on Islamabad's Red Mosque to dislodge a pro-Taliban movement killed 102 people.

The day began with a 31-gun salute in Islamabad and 21-gun salute in the capitals of Pakistan's four provinces.

Thousands of people gathered for a midnight fireworks display in Rawalpindi, called Islamabad's twin city, and in the city of Lahore troops hoisted the national flag at the Wagah border with India as some 200 people raised "Long live Pakistan" slogans.

But a bomb hoax near the mausoleum of Pakistan's founder, Mohammad Ali Jinnah, worried hundreds of people who had come there to celebrate Independence Day in the port city of Karachi.

"I come here every year with my family, but there was a fear this time due to bomb threats across the country," said Mohammad Yaqoob.

Explosives detonated in Karachi damaged a main transmission line tower, while police were able to remove explosives from two other power pylons. It was unclear who placed the explosives.

In Islamabad, which in past years would have been decked out with flags and illuminations, Independence Day was a low key affair due to security fears.

Police checked vehicles in and around the capital where two suicide bombings killed at least 26 people last month.

Security concerns kept many people indoors in Peshawar, the provincial capital of the North West Frontier Province whose tribal areas have been a hotbed of al Qaeda and Taliban support, and Quetta, capital of the southwestern Baluchistan province.

NO TO "FOREIGN POWERS"

Pakistan's Prime Minister Shaukat Aziz vowed on Tuesday to stop any "foreign power" from violating the country's borders, in an apparent reference to statements from U.S. politicians doubting whether Pakistan was making progress against militants.

"I want to make it clear that not under any circumstances will we allow any foreign power to enter Pakistan's territory," Azuz said at a traditional flag-hoisting ceremony in Islamabad to mark Independence Day.

Some U.S. politicians recently said the United States must be willing to strike al Qaeda targets in Pakistan even without Islamabad's permission -- drawing rebuke in the country.

President George W. Bush also signed into law a bill requiring him to confirm Pakistan's progress in fighting Taliban and al Qaeda on the Afghan border before releasing future aid, disturbing Pakistanis sensitive to what they see as excessive U.S. demands.

Newspapers were packed with opinion pieces analyzing Pakistan's 60 years. Many focused on what one commentator called an "orgy of pessimism" surrounding Pakistan's troubled years of military rule and struggles with democracy.

"True, we have made blunder after blunder, committed terrible crimes against our own people," Dawn newspaper said in one editorial.

"All said and done, there has been progress, though, admittedly, the rate could have been faster."