Uganda has deployed the army and police as fire has destroyed much of the Kasubi Tombs, the final resting place of royalty and a UNESCO World Heritage Site dating back 150 years in protests by youths who claimed it was arson.
Anti-riot units battled during the night to disperse young supporters of the Ronald Muwenda Mutebi II, traditional ruler of the Baganda, one of Uganda's main tribes.
The cause of the blaze which started early on Tuesday night and which consumed the thatched-roof mausoleum and many royal artefacts of the Bugandan kingdom has not been identified.
"Our officers are at the scene trying to examine every available evidence and piece together bits of information to establish what clearly happened but no conclusion has been reached yet," said police spokeswoman Judith Nabakooba.
The Baganda are Uganda's largest tribe and were instrumental in Ugandan President Yoweri Museveni coming to power 24 years ago. Museveni based his five-year military struggle in the kingdom's heartland and support by the Baganda helped him stay in power.
A kingdom official, Charles Peter Mayiga, said the gutted mausoleum was built in 1860 by Kabaka Mutesa I and that its historical and cultural significance was an object of deep reverence by the Baganda.
The kingdom's leadership was meeting to try and establish what had caused the fire, he said.
"There are people who want to cause harm to this kingdom. They are keen on destabilising us and we don't know whether they're behind this," he said.
A local paper daily said arson could have sparked the flames.
"A white numberless pickup truck reportedly emerged from the tombs shortly after the fire broke out," the paper said, quoting motorcycle taxi riders near the scene. Other witnesses, the paper said, reported a fleeing man shooting in the air to scare away motorcycle riders pursuing him.
President Yoweri Museveni is expected to inspect the damage on Wednesday
Bloody riots ensued in September, which left scores of people dead. Authorities also shut down the kingdom's radio station for allegedly fanning violence.