The attack began around midday Saturday, when the robbers threw grenades into the shopping mall. The Westgate Mall is popular with wealthy Kenyans and expatriates living in the Kenyan capital.
"I could hear the gunfire moving towards the main entrance of the shopping mall, so some people ran out of our cafe in a kind of panic, and quite a number just fell down as flat as possible on the ground," Arjen Westra, who escaped the attack, told the BBC.
The stand-off continues this afternoon as police have surrounded the mall and are attempting to free shoppers who have taken cover, and shoppers who are now being held hostage.
The BBC received the following emails from a shopper taking cover in the mall:
11:33: Am hiding in a store, my next person (is) an Indian shot. Severe shooting going on within the premises. I left (a) parcel in my car mid of the road, praying.
12:28: Thugs inside, we don't know when the police will rescue us, all over TVs and radio, we are warned not to move, am in a dark store, more police coming.
12:38: I don't even know if I stopped engine of car. Nobody is going out, only police. Praying they ditch thugs out. It is a big shopping mall, now knowing where thugs are is hard. Getting out of here is a mystery. My next person (is) an Indian shot and bleeding.
We'll continue to update throughout the day.