Amber and yellow weather alerts for heavy downpours are in place across large parts of Scotland.
Ten motorists were airlifted to safety after drivers were trapped near Inveraray, Argyll and Bute, Police Scotland said.
The A83 Rest and Be Thankful was hit by seven landslides during extreme weather, and the Scottish Government held crisis talks in anticipation that the situation would get worse.
Preparations were made to open community centres in Argyll and Bute to help vulnerable people, while Police Scotland warned against travelling to the region.
Kayakers took advantage of floodwater in Inverclyde and paddled down submerged streets, and around 12 cars were spotted abandoned in floodwater in the coastal region, which experienced "Biblical" rainfall overnight.
Train services were cancelled or finished early at 9pm, and motorists were advised to delay travelling if possible.
A spokesman for ScotRail said: "Some areas have seen up to a month's worth of rain in a 24-hour period resulting in heavy flooding."
The Met Office issued an Amber warning - meaning "danger to life" - across Angus, Perth and Kinross, Aberdeenshire, Moray and Highland until 2pm on Sunday.
Other parts of Scotland that were badly impacted included Renfrewshire, East Renfrewshire, Glasgow, North Lanarkshire, East and West Dunbartonshire, Argyll and Lochaber, parts of Stirling, Perthshire, Aberdeenshire and the Highlands.
Debris was swept across the road networks, and a clean-up operation on the A83 has yet to begin as it was deemed unsafe.
Police Scotland Assistant Chief Constable Tim Mairs said: "Officers have been dealing with significant disruption on the roads caused by the heavy rainfall and we would expect this to continue into Sunday.
"Our advice is to plan ahead and consider if your journey is really necessary or if it can be delayed until conditions improve."
Vincent Fitzsimons, flood duty manager for the Scottish Environment Protection Agency (Sepa), said: "I want to be very clear that this is not a normal autumn day for Scotland. We're expecting extensive river and surface water flooding in affected areas. There is a danger to life.
"There will be widespread impacts to road and rail. There is a risk of more significant community scale property flooding."
Justice Secretary and Minister for Resilience Angela Constance chaired a meeting of the Scottish Government's Resilience Committee (SGoRR), which was attended by First Minister Humza Yousaf.
Ms Constance said: "As the weather warnings outline, heavy rainfall is expected to continue in many parts of the country into Sunday and some areas will have a month's worth of rain over the course of the weekend.
"I would urge everyone across the country to heed the travel warnings being issued by Police Scotland and others - in particular, drivers in Argyll and Bute should avoid travel due to the significant disruption across the road network.
"We are aware that the initial, most severe impacts have been felt by people and businesses in Argyll and Bute, as well as other areas in the west and north of the country.
"Ministers are receiving regular updates on the situation from partners, including Sepa, the Met Office, and Police Scotland as it unfolds.
"This afternoon I convened SGoRR in recognition of the severity of the situation which will continue to develop over the next 24 hours."