|Huge boulders were among the hundreds of tons of earth that came down on Sugar Loaf Mountain|
Tearfund says it has been told that there are churches where dozens of members of the same congregation have died.
Sky's Rebecca Williams, in the capital Freetown, says she has been told that more than 500 bodies have now been brought to the city's morgue.
Storms swept through the capital of the West African country on Monday, leaving streets under water and causing whole hillsides to collapse in rural areas.
Gaston Slanwa, Tearfund's Country Representative for Sierra Leone said: "I drove around Freetown yesterday. On one bridge I saw two people who were already dead being pulled out of the water.
"In just one of the church communities we work with, 60 people have died and 300 have lost their homes."
ActionAid said it had been told that, so far, 3,400 people have been reported missing.
The charity's Zynab Kamara said: "In nearly a decade of working on humanitarian response I've never witnessed anything so tragic.
"I saw children clambering over the rubble and mud, desperately calling for their parents."
A resident looks down on the area where thousands of homes have been flattened and buried
Sierra Leone's president Ernest Bai Koroma issued a desperate appeal for help, fighting back tears as he toured Regent, one of the worst-hit areas.
|People wearing protective suits hold hands as they cross a river near one of the landslides|
A spokesman for the UK's Department for International Development (DFID) said British military personnel were able to assist at the scene within just hours of the landslip.
DFID said it was providing help with healthcare, water, sanitation, humanitarian supplies, generators and tents and in drawing up "a cholera response plan" and a British firm Dawnus has offered specialist equipment to help with recovery.
People wearing protective suits hold hands as they cross a river near one of the landslides
The UK has had a strong relationship with Sierra Leone for some years, regularly posting members of DFID and the MoD in the country since British military forces intervened in the civil war in 2000 and following the Ebola crisis in 2014.
The UN said it was evaluating needs in the country and that "contingency plans are being put in place to mitigate any potential outbreak of waterborne diseases such as cholera, typhoid and diarrhoea".
Mr Koroma's office pleaded with residents to go to the morgue quickly to identify their dead loved ones otherwise all unidentified corpses will be given a "dignified burial" in mass graves.