Whitney Houston -- Casket Under LOCKDOWN Until Burial

The casket of Whitney Houston will be heavily guarded until the burial Sunday morning, with a whopping 10 security guards tasked with keeping watch on it ... TMZ has learned.

According to our sources, the casket will be kept at the cemetery until tomorrow's burial ... but will be well-protected until then.

We're told the family will get an opportunity to see Whitney one more time between now and tomorrow morning.