One Pakistani navy officer and a fire fighter were killed as explosions and gunfire rocked the base, said navy spokesman Irfan ul Haq said. Many ambulances were held back because of the fighting that was still going on more than 2 1/2 hours after the attack began on the Mehran naval station.
The coordinated strike by between 10 to 15 attackers came just under three weeks after the death of Osama bin Laden in an American raid on the northwestern garrison city of Abbottabad, an event al-Qaida allied extremists here have vowed to avenge. The unilateral American raid triggered a strong backlash against Washington, which is trying to support Pakistan in its fight against militants, as well as rare domestic criticism against the armed forces for failing to detect or prevent the operation.
The fact that militants were able to enter one of the country's largest military bases is another embarrassing blow to the army and will raise questions over whether the attackers had inside information. That they targeted a U.S. supplied aircraft draws attention to American aid to the military, something generals here do not talk about, fearing criticism from the county's fiercely anti-American people.
After heavy American prodding, security forces launched several operations against militants in their heartland close to the border with Afghanistan over the last three years. The extremists have struck back against police and army targets around the country.
Sunday's raid appeared to be most serious against the military since October 2009, when militants attacked the army headquarters close to the capital, Islamabad. They held dozens hostage in a 22-hour standoff that left 23 people dead, including nine militants.
It began with at least three loud explosions, which were heard by people who live around the naval base, one of the largest military facilities in the country. It was unclear what caused the explosions, but they set off raging fires that could be seen from far in the distance.
An Associated Press reporting team outside the base heard at least six other explosions and sporadic firing.
Authorities sent in several dozen navy and police commandos to battle the attackers, who responded with gunfire and grenades, said a second navy spokesman, Salman Ali. At least one airplane - a P-3C Orion, a maritime surveillance aircraft that was recently given to Pakistan by America, - was destroyed, he said.
The United States handed over two Orions to the Pakistan navy at a ceremony at the base in June 2010 attended by 250 Pakistan and American officials, according to the website of the U.S. Central Command. It said by late 2012, the Pakistan would have eight of the planes.
At least one media report said team of American technicians were working on the aircraft at the time of the strike, but U.S. Embassy spokesman Alberto Rodriguez said no Americans were on the base. Ali also stated there was no foreigners inside the base.
No group claimed responsibility for the attack. But the Pakistani Taliban, an al-Qaida allied network which has previously launched attacks in Karachi, has pledged to retaliate for the death bin Laden, and has claimed responsibility for several bloody attacks since then.
Pakistani Prime Minister Yousuf Raza Gilani condemned the attack, saying such a "cowardly act of terror could not deter the commitment of the government and people of Pakistan to fight terrorism."
Karachi, the country's largest city and its commercial hub, has not been spared the violence sweeping the country, despite being in the south far from the northwest where militancy is at its strongest.
In April, militants bombed three buses taking navy employees to work, killing at least nine people.
The Pakistani Taliban and other militant groups have little direct public support, but the army and the government have struggled to convince the people of the need for armed operations against them. The militants' identification with Islam, strong anti-American rhetoric and support for insurgents in Afghanistan resonates with some in the country.