Four men, one woman and a seven-year-old girl died in the landslide, according to the National Disaster Management Agency (NDMA).
Two men were also killed in the Mount Aureol and Blackhall Road neighbourhoods when fences collapsed onto buildings, the agency said.
In the Colbert community, more than 800 people were displaced by flash flooding, Mohammed Bah, an NDMA spokesman, told AFP.
"The landslide was due to the heavy rain, no doubt, but also a combination of illegal human activities", Bah said.
"People are cutting down trees, tampering with the forest cover... The mudslide that occurred is mainly a result of the fact that people are building (houses) beyond demarcated zones."
Authorities have asked residents to evacuate the an area of Looking Town, which is surrounded by hills, due to a "very big boulder" that is poised to fall.
President Julius Maada Bio blamed the disaster partly on climate change.
"The heavy downpour experienced this August points to the impact and consequence of global warming and climate change", he tweeted on Sunday night.
"But years of poor urban planning and mismanagement of the city's resources are an enormous contributor to flooding across Freetown".
The World Food Program said on Twitter it is supporting the NDMA's emergency response efforts.