President Uhuru Kenyatta said 67 people died, including six security personnel.
Five militants were killed and 11 suspects arrested, he said.
Islamist group al-Shabab, which claimed responsibility for the attack, said 137 hostages had died. But the statement cannot be verified.
As the clearing of the mall continues, the death toll is expected to rise.
Several bodies, including those of some attackers, are thought to be trapped under rubble after three floors of the building collapsed following a blaze.
Kenya’s Standard newspaper reported that tens of bodies were removed from the building on Tuesday evening.
Correspondents say the shopping centre lay largely silent overnight and light smoke was still drifting from the building.
Journalists and onlookers were kept behind a security cordon but police let some people retrieve cars from the scene.
In his address, President Uhuru Kenyatta praised the response of ordinary Kenyans, calling it exemplary and overwhelming.
“We have ashamed and defeated our attackers,” he said.
“Kenya has stared down evil and triumphed.”
He did not confirm earlier reports that several of the attackers were American and British.
“Intelligence reports had suggested that a British woman and two or three American citizens may have been involved in the attack,” said Uhuru Kenyatta.
“We cannot confirm the details at present. Forensic experts are working to ascertain the nationalities of the terrorists.”
He added: “These cowards will meet justice, as will their accomplices and patrons, wherever they are.”
According to the UK Foreign Office, one British national had been arrested in Nairobi, without giving further details.
One of the people arrested is understood to have been in the shopping centre, though it is not clear whether they were armed, or among the 10-15 attackers that Kenyan authorities have spoken of.
At least 18 foreigners are among the dead. About 175 people were wounded, including 62 who remain in hospital.
Somali Islamist group al-Shabab said it had carried out the attack in retaliation for Kenyan army operations in Somalia.
The militants stormed the Westgate centre on Saturday, throwing grenades and firing indiscriminately on shoppers and staff.
Twitter posts from al-Shabab on Wednesday said it had held 137 people hostage, and claiming they died at the hands of the security forces at the end of stand-off.
Al-Shabab, which is linked to al-Qaeda, has repeatedly threatened attacks on Kenyan soil if Nairobi did not pull its troops out of Somalia.
There are about 4,000 Kenyan troops in the south of Somalia as part of an African Union force supporting Somali government forces.
Al-Shabab is fighting to create an Islamic state in Somalia.
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