NAIROBI (AFP) - Masked attackers stormed a packed upmarket shopping mall in Nairobi on Saturday, spraying gunfire and killing 30 people and wounding dozens more before holing themselves up in the complex.

Kenyan security forces said have arrested one of the gunmen, the Kenyan Presidency said on Twitter.

The gunmen were "pinned down" after hours of painstaking evacuations, with police going shop to shop to secure the Westgate shopping mall, a security source told AFP.

"The attackers have been isolated and are pinned down in an area on one of the floors. The rest of the mall seems to be secure," a security source told AFP at the scene.

Senior police sources said they believed a well-organised "terror gang" numbering around 10 was behind the assault on the shopping centre, which was packed with around 1,000 shoppers when it was besieged at midday.

An eyewitness told AFP that he heard the gunmen speaking Arabic or Somali and saw the group executing shoppers, in what appeared to be the worst attack in Nairobi since an Al-Qaeda bombing at the US embassy killed more than 200 in 1998.

"The death toll is now standing at 30. This includes those who have died at the scene and at the hospital," a senior police official told reporters. The Red Cross confirmed the figure and said another 60 had been wounded in the attack.

The Kenyan government, which has troops battling Islamist insurgents in neighbouring Somalia, said it was too early to say who was responsible.

"Investigations have begun to find out the perpetrators of this crime. I urge Kenyans not to speculate," Interior Minister Joseph Ole Lenku said in a statement.

Police at the scene said a suspect wounded in the firefight had been detained and taken to hospital under armed guard.

Earlier a police source said it had been confirmed that the attackers were holding at least seven hostages. As darkness fell over Nairobi, their fate was unclear.

Kenyan troops could be seen moving around and inside the shopping centre while special forces had joined the operation. Dozens of wounded, some of them bleeding children, were taken away from the mall on stretchers.

The four-storey mall, which has several Israeli-owned businesses, is a hub for Nairobi-based Westerners and one of the foremost symbols of Kenya's affluent classes. It has long been considered a potential terror target. It opened in 2007 and has restaurants, cafes, banks, a large supermarket and a cinema.

It is popular with the large expatriate community living in the residential neighbourhoods around it, including with foreign staff from the United Nations, which has its third largest global centre nearby.

Security agencies have regularly included the Westgate shopping centre on lists of sites they feared could be targeted by Al Qaeda-linked groups.

The Somali insurgents from the al Shabaab group have repeatedly threatened to strike at the heart of Kenya in retaliation for Nairobi's military involvement alongside the government they are trying to overthrow.

UN Secretary-General Ban Ki-moon is following the attack "closely and with alarm", a statement from his office said.