"This is terrorism... this is a heinous act, we are under threat but we will not be cowed," Odinga told reporters at the site of the blast in Nairobi's commercial heart.

Kenya has been hit by a wave of grenade attacks the police have repeatedly blamed on Somalia's Al-Qaeda linked Shebab insurgents or its supporters.

Last month the hardline Shebab warned Kenya of revenge attacks for sending tanks and troops into Somalia in October.

The blast, which ripped through a small shopping complex on Nairobi's Moi Avenue in the early afternoon, left around 30 people wounded.

Witness blames bearded man

A witness wounded in a blast in downtown Nairobi says that the explosion occurred soon after a bearded man left a bag near her stall.

Irene Wachira spoke to The Associated Press from a Nairobi hospital bed on Monday. Wachira said the man came to the stall three times and acted as if he were interested in buying something. Wachira said the third time he came with a bag, he left it behind and then there was a blast.

Wachira described the man as "Arabic-looking" because of his relatively light skin. A doctor told AP that another person wounded in the blast said a Somali-looking man left behind the bag.

Minutes after the blast the pro-Shebab Twitter site Al-Kataib reported a "huge explosion in Nairobi" although did not specifically claim any responsibility.

"Kenya under attack? Of course the kuffar (unbelievers) has attacked Somalia!" read another Twitter message from the Kenyan-based Muslim Youth Centre, a group linked to the Shebab, according to United Nations reports.

Powerful blast

The blast was powerful enough to rip the tin roof off the building and smash windows across the street.

Wounded were carried from the explosion in a commercial building housing several small shops. Many appeared to have cuts from broken glass or flying debris.

Police Commissioner Mathew Iteere had earlier said the explosion was not a grenade or bomb attack, and that "preliminary conclusions" suggested it was an electrical fault.

A large plume of black smoke rose high into the air as ambulances rushed the wounded to hospital, but fire fighting trucks with sirens blaring extinguished the blaze soon after.

"I addressed parliament and warned of such acts... we chased them from here now they are trying to come back," Odinga added.

"We'll remain united. Our security forces need to be strengthened," he added.