The waters swept away homes and left residents desperate for news of missing family members.
250 bodies have been recovered so far, and police say they are yet to find any survivors.
Torrential rain caused a hillside to collapse on the outskirts of the capital, Freetown, burying dozens of homes in mud.
Many people living at the foot of Mount Sugar Loaf were asleep at the time.
Scores drowned in their homes, while others were swept away.
Police and military personnel remain at the scene.
Sierra Leone Police director of operations Al-Sheik Kamara says authorities scrambled to coordinate a response.
"Because, this kind of slide that we are experiencing now is so unfortunate, that a lot of lives have been lost here, we have called on the police to cutting off the area so that we can call in rescue workers, the military is here, the ONS, we are all partnering , we have established a command centre very close to the epicenter here, and we are coordinating effort to make sure we get people out, we have taken a lot of dead bodies, we have not so far taken out anybody alive."
Journalist Umaru Fofana is based in Freetown, and says he witnessed people digging in the hope of finding their families.
"I went down to the spot myself and you could see people using their bare hands, pulling up corpses from beneath the mud. The road itself, so the disaster area, is almost impassable, massive rocks. And this area called Mount Sugar Loaf, the mountain caved in the early hours of this morning, and it's covered, literally, dozens of houses, and hundreds of people according to the country's vice-president who just spoke to me, are feared dead under the rubble. So there are some ambulances that are parked here, but it's practically become a recovery mission instead of a rescue mission."
A high volume of bodies continue to be recovered from underneath the rubble, with the death toll expected to rise.
Freetown's city morgue says bodies have had to be stored in corridors.
President Ernest Bai Koroma says it is the biggest natural disaster to have hit the country, ever.
"I am very disturbed by this national tragedy and with a heavy heart let me extend profound condolences to the bereaved families. This is not a tragedy for you alone, it is a tragedy for every civilian because the people who have perished in this disaster are our compatriots."
The devastation has been blamed on a catastrophic mix of poor planning, deforestation, and heavy rains.
Freetown is an over-crowded coastal city with few defences against such wild weather.
British charity Home Leone is helping build stronger houses in Sierra Leone.
Chief Executive Nigel Hyde explains many victims lived in poorly constructed homes, often shacks on unprotected hillsides.
"People build houses all up the sides of cliffs, and they often build them with inadequate materials because generally people are unbelievably poor. People are trying to reclaim from the sea, and then water comes and just wipes them away."
Sixty per cent of people in Sierra Leone live below the poverty line.
President Koroma says all society classes need to unite in support of the victims.
"Let me assure you that my government is fully engaged on this situation and in collaboration with our development partners we have already established an emergency response centre at Regent to coordinate our response and to provide relief to the survivors."
