Twenty-seven people were killed and more than 50 wounded when a series of explosions hit the centre of Afghanistan's southern city of Kandahar after nightfall on Saturday, a hospital official said.

Officials said the biggest attack was aimed at the prison in the city, possibly an attempt to repeat a brazen jailbreak there two years ago. The city is at the centre of the Taliban's heartland and the next major target for NATO forces this year.

Abdul Qayyum Pukhla, head of Kandahar's main hospital, said the 27 dead and 52 wounded included both police and civilians.

The first explosion was near the police chief's compound and the second near the residence of President Hamid Karzai's half-brother Ahmad Wali Karzai, who is the head of Kandahar's provincial council, a Reuters witness said.

Ahmad Wali Karzai, who was in the capital Kabul, said the biggest explosion was a suicide strike at Kandahar's prison, where he believed insurgents were trying to release prisoners.

"The main target was the prison. The prison is very well guarded," he told Reuters. "We don't know about the casualties. It was a very big explosion. It was a huge explosion."

Taliban insurgents stormed the prison on the outskirts of the city in 2008, freeing around 1,000 prisoners, including 400 Taliban fighters.

Two of the other blasts took place near the provincial council building, including one about 200 metres (yards) from his home, and another struck near the police chief's headquarters, Ahmad Wali Karzai said.

He said the other blas

ts appeared to be diversions designed to draw police away from the main attack on the prison.

There were at least three explosions in the city, and Western troops were assisting at the request of Afghan authorities, said Major Macen Waoczak, a spokesman for the NATO-led International Security Assistance Force.

Police at the scene of the first blast were shooting, a Reuters witness said. Windows in nearby buildings were shattered.

A police source in Kabul said four suicide bombers had attacked Kandahar and had also fired rockets into the prison. Afghan police and Afghan special forces were deployed, and there had been an exchange of gunfire, the police source said.

Kandahar, Afghanistan's second city, has been the scene of numerous Taliban attacks. U.S.-led forces plan to launch an operation this year to take back the city and surrounding areas, which have increasingly been under the insurgents' control.

Kandahar was the spiritual homeland of the Taliban when the strict Islamists ruled Afghanistan in the 1990s, and the U.S. and NATO commander, General Stanley McChrystal, considers it the main geographical target of the Taliban leadership.

It is also the home town of the Karzai family.

The bulk of 30,000 additional combat forces ordered to Afghanistan by U.S. President Barack Obama at the end of last year are expected to be deployed in Kandahar as part of the major operation over the next few months. Thousands of Canadian troops also patrol the city.

(Additional reporting by Peter Graff and Hamid Shalizi in Kabul; Writing by Jonathon Burch and Peter Graff; Editing by Jon Hemming)