Gunmen armed with rockets and explosives stormed a major Pakistani naval air base, triggering gunbattles that killed two navy staff.
The raid comes three weeks after the US killing of Osama bin Laden.
Around 10 people were wounded and towering flames rose over PNS Mehran in the centre of Karachi, Pakistan's largest city, where the military and government confirmed that the base was under "terrorist attack".
An AFP reporter saw swarms of soldiers and navy commando reinforcements pile into the base as smoke rose into the night sky. Over a period of three hours, an AFP photographer heard seven blasts and periodic bursts of gunfire.
There was no claim of responsibility but Pakistan's military has long been on the frontline of attacks blamed on the Taleban and other Al-Qaeda-linked militant groups that have killed more than 4,350 people in four years.
The Taleban have recently stepped up threats against Western and Pakistani government targets to avenge the killing of bin Laden by US Navy SEALs in the garrison city of Abbottabad near the capital Islamabad on May 2.
Officials said at least 10 militants crept up to the base on three sides, using the cover of night to approach seemingly undetected through neighbouring civilian residential areas and through trees and foliage.
"The attackers first fired rockets," Navy spokesman Commander Salman Ali told ARY television station, denying any staff had been taken hostage but conceding that a long-range Orion aircraft had been destroyed.
Last June, the United States delivered two P-3C Orion maritime patrol aircraft to PNS Mehran.
"The terrorists also used small bombs and now they are firing with sophisticated weapons. They are inside and still resisting. They have destroyed an aircraft," he added, around three hours after the attack began.
Prime Minister Yousuf Raza Gilani condemned the assault, and ordered his interior minister to Karachi to "coordinate the security efforts being taken by the civil and military officials," his office said in a statement.
Commodore Irfan ul Haq, another spokesman for the Pakistan Navy, told AFP that one Navy officer and another staff member had been killed.
Home ministry official Sharfuddin Memon from the southern province Sindh said "more than 10 terrorists" were inside the base and at least 10 people had been wounded, but could provide no further details.
In October 2009, Taleban militants besieged the army headquarters in the garrison city of Rawalpindi for two days, killing 22 people and raising serious questions over why it took the military so long to put down the assault.
Karachi, Pakistan's financial capital whose sea port is used by NATO to ship supplies to the estimated 130,000 US-led foreign troops fighting the Taleban in neighbouring Afghanistan, has recently seen a spike in attacks on the military.
On April 28, four naval personnel and a passing motorcyclist were killed in a bombing, two days after four others were killed in navy bus bombings.
Last week, a Saudi diplomat was shot dead as he drove to the Saudi consulate in the city of 16 million people, just days after attackers threw grenades at the mission.
Pakistan's seemingly powerful security establishment was left humiliated by the discovery and killing of the Al-Qaeda terror chief in a unilateral American Navy SEAL raid that has rocked relations with wary ally Washington.
In an interview with the BBC broadcast on Sunday, US President Barack Obama said he stood ready to order a similar mission to that which killed bin Laden if another high-value target was discovered in Pakistan, or any other country.
"We are very respectful of the sovereignty of Pakistan, but we cannot allow someone who is actively planning to kill our people or our allies' people, we can't allow those kinds of active plans to come to fruition without us taking some action," he added.
On Sunday, thousands of people demonstrated in Karachi to demand an immediate end to US missile strikes in Pakistan's northwestern tribal belt on the Afghan border and call for a block on NATO supplies passing through the country.
Activists from the Tehreek-e-Insaf (Movement for Justice) led by former cricket hero Imran Khan held a two-day sit-in outside the Arabian Sea port, urging the government to end its cooperation with Washington's "war on terror".