Several people are feared dead and many more wounded after militant extremists stormed a Nairobi hotel.
Authorities have not released a death toll, but witnesses have reported seeing up to five bodies.
Survivors described a shattering explosion and people shot to death while sitting at a cafe.
Kenya's national police chief Joseph Boinnet said a blast in the lobby severely injured a number of hotel guests.
A local hospital has appealed for blood donations to help with the wounded.
The complex in Nairobi’s Westlands neighbourhood includes a large hotel known as DusitD2, banks and offices.
Several vehicles were burnt and people were rushed or carried from the scene.
The al-Qaida-linked al-Shabaab extremist group has claimed responsibility for the attack which happened a short distance from the Westgate Mall, which was attacked by the extremist group in 2013, killing 67 people.
Enoch Kibet, who works as a cleaner at the Secret Garden cafe, said: "We were changing our shifts and that is when I heard a loud blast and people were screaming." She crawled out of a gate in the basement.
She says that "I couldn't believe I was alive. The blast was so loud and shook the whole complex."
A waitress at the same cafe who only gave her first name, Lily , trembled as she described people lying on tables bleeding.
"It is terrible. What I have seen is terrible. I have seen a human as I ran out and there is what looks like minced meat all over," said one man who said he ran from the scene, Charles Njenga.
"I have been hiding. My colleagues were running everywhere,” said another man.
Gunfire continued several minutes after the first reports as black smoke rose from the scene.
"We have sent officers to the scene, including from the anti-terrorism unit, but so far we have no more information,” police spokesman Charles Owino said.
Groups of people were hurried away as plainclothes officers went shop to shop in the complex.
What appeared to be plainclothes security forces inched their way towards the scene with guns as helicopters could be heard overhead. Other people appeared to be taking cover behind fountains and other features in the outdoor complex.
This incident is the latest in a string of terrorist attacks in recent years and comes three years to the day al-Shabaab extremists attacked a Kenyan military base in neighboring Somalia, killing scores of people.
In April 2015 al-Shabaab claimed responsibility for an attack on Garissa University that killed 147 people, mostly students.
The attack comes a day after a magistrate's court ruled that three men must stand trial on charges they were involved in a deadly attack on the Westgate shopping mall in 2013.
The al-Qaida-linked al-Shabaab objects to the presence of Kenyan troops in the turbulent Horn of Africa nation.
In a tweet, foreign secretary Jeremy Hunt said he was "shocked" at the "dreadful scenes in Nairobi" and offered his condolences to those caught up in the attack.
He added: "Brits in the area should remain vigilant and follow advice of local security authorities" and advised travellers to follow foreign office updates.
Currently, most of Kenya is considered safe, but the foreign office warns against all but the most essential travel within 60km of the Kenya-Somali border.