A loud blast and gunshots have been reported in 14 Riverside drive in Westlands, a source told Mkarimu Media.
Reports indicate there was an explosion outside one of the buildings before gunshots filled the air at around 3PM, East Africa time.
Three vehicles could be seen burning in the area.
People trapped inside the buildings report of incessant shootings with many taking to social media to say they are holed up in several of the adjacent buildings. Many others reported of an active shooter in one of the buildings.
Ronald Wafula, a resident from the area said they were under attack from unknown people.
There is an on-going police operation along that road and motorists are encouraged to avoid the area.
Officers at the scene said the attackers came with a vehicle wearing a bullet proof vest ready to attack.