KANDAHAR, Afghanistan - The Taliban unleashed a major assault Saturday on government buildings throughout Afghanistan's main southern city, an attack that cast doubt on how successful the U.S.-led coalition has been in its nearly yearlong military campaign to establish security and stability in the former Taliban stronghold.

The Taliban said their goal was to take control of Kandahar city, the birthplace of the Taliban and President Hamid Karzai's home province, making it the most ambitious of a series of recent high-profile attacks on government installations. The attack came a day after the Islamic movement said Osama bin Laden's death would only serve to boost morale, but a Taliban spokesman insisted it had been in the works for months before the al-Qaida leader was killed by American commandos on Monday.

Shooting started shortly after midday and lasted more than seven hours, while government forces were backed by military helicopters firing from overhead.

At least eight locations were attacked: the governor's compound, the mayor's office, the intelligence agency headquarters, three police stations and two high schools, according to government officials.

The assailants included at least five suicide attackers in bomb-rigged cars, three of whom were stopped by police before their explosives could go off, NATO forces said in a statement. In the end, none of the assaulted compounds was breached by the militants, NATO said.

About 40 to 60 insurgents were involved in the assault, according to U.S. Army Lt. Col. Webster Wright, a spokesman for NATO's Regional Command Southeast. The rebel force was split into squad-sized units and may include more suicide bombers, Wright said.

"They're trying to make a spectacular event," he said.

The attackers at the governor's compound were finally pushed back around nightfall and Gov. Tooryalai Wesa called reporters in for a press conference at his reclaimed office while fighting continued at the intelligence agency a little over a mile (a kilometre) away. Heavy gunfire finally died down across the city around 8:30 p.m., though sporadic shooting continued in the area around the intelligence agency.

At least one police officer and one civilian were killed and 20 other people wounded in the assaults, Gov. Tooryalai Wesa told reporters in a news conference at his reclaimed office, adding that the death toll was likely to rise as troops searched the area.

He said six Taliban fighters also have been killed.

The Taliban said more than 100 militants flooded into the city -- including many escaped convicts who had been freed in a bold Taliban prison break last month. They were told to target any building used by the government or security forces.

"We are taking control of the entire city. We are at every corner," Taliban spokesman Qari Yousef Ahmadi told The Associated Press in a phone call.

The Taliban usually exaggerate the scale of their attacks, and it is unlikely the movement would have the strength or the numbers to actually take over Kandahar. A NATO official, who spoke on condition of anonymity in order to let the Afghan government make official statements, said the insurgents have not controlled any part of the city during Saturday's assaults.

But Wright, the U.S. Army spokesman, said the fighting would likely continue into the morning and that it was a show of strength by the Taliban, who recently announced their spring offensive.

"They're attacking government institutions and trying to say that that the government cannot defend itself," he said.

Wright said part of the insurgents' strategy was to draw NATO airstrikes, which could cause collateral damage in the densely populated provincial capital.

The battle is a critical test of mettle for Afghan forces in Kandahar city, where insurgents recently killed the chief of police in a suicide bombing and released 480 prisoners from the city's prison through an underground tunnel that had been dug over several months.

Wright said that NATO strengthened perimeter security around two police buildings, but otherwise exercised minimal force during Saturday's clashes.

Saturday's attack shows the resilience and determination of the insurgency in the face of a massive international push to remove the Taliban permanently from the city that was once their capital. Government officials said they had no accurate estimate of how many attackers were involved.

The persistent violence has complicated the situation for U.S. and many NATO allies who are hoping to pull out troops. President Barack Obama wants to start drawing down forces in July and the alliance has committed to handing over control of security in the country to Afghans by 2014.

President Karzai expressed belief that the attack was an effort to avenge bin Laden's death and called it reprehensible.

"Al-Qaida terrorists have experienced a major defeat in the death of Osama bin Laden in Pakistan and now to hide that defeat they are attacking Kandahar and killing civilians. They are trying to get their revenge from innocent Afghan people," Karzai said in the statement. He did not mention the Taliban by name in the statement.

Ahmadi said the Kandahar plot had been in the works for months and was not a revenge attack for bin Laden's death. The Taliban have promised more large attacks as part of a spring offensive.

In Kabul, a spokesman for the intelligence agency insisted that Afghan security forces were still in command of the area.

"The police are in control of Kandahar city," agency spokesman Latifullah Mashal said.

Residents did not appear convinced. Shopkeepers throughout the city closed down their stores and the streets emptied of people and cars as Kandahar residents bunkered down to wait out the fight.

"We were eating lunch when suddenly the shooting started," said 20-year-old Sayedullah, who lives in the city. "No one can go out because the fighting is still going on. The situation is very bad." He said security forces had closed all the roads so he couldn't go outside if he wanted to.

NATO troops -- most of them American -- have poured into Kandahar over the past year as part of a plan to route the Taliban from their southern strongholds and establish enough security to prevent them from returning with their usual force this spring. NATO has also helped dramatically increased the number of police in Kandahar city and its environs, and offered them more training.

International military officials have said that the Taliban have now been significantly weakened by a winter of heavy fighting and by the loss of weapons caches that they usually return to each spring as the fighting season picks up.

But the Taliban have responded with assassinations, suicide bombers and attacks on high-profile officials.

The insurgents have also launched deadly attacks recently from inside the Defence Ministry in Kabul and from inside a joint U.S.-Afghan base in the east.

"We have expected that the insurgents would try to re-infiltrate the places where they had free reign," coalition spokesman Lt. Col. John Dorrian said. "We do not see this as something that has any lasting or strategic impact."