The Forth Road Bridge is to be closed until the new year because of structural faults, Transport Minister Derek Mackay has said.
The bridge has been closed since midnight with initial guidance saying it would be shut for at least 24 hours.
The minister has now said it would need at least three weeks to repair the bridge and safety was "paramount".
He said a full travel plan was being prepared, including extra trains and buses and possibly a ferry.
Early on Friday morning, 11-mile tailbacks were reported approaching the Kincardine Bridge, the alternative route across the Forth from Fife to Edinburgh.
Heavy congestion was reported on the same routes on Friday evening.
At a media conference, engineers said a 20mm-wide crack in a truss under the southbound carriageway close to the bridge's north tower could not have been predicted and happened quickly.
They said continuing to allow traffic to use the bridge would "increase the risk of causing extensive secondary damage to the structure".
The fault was not on the bridge during the last inspection in May and was only spotted on Tuesday. Officials said the shear fault was discovered in the most difficult area of the bridge to access.
Bridge operator, Amey, said it was an "unprecedented situation".
Traffic Scotland has said all current and planned roadworks on the diversion routes for the bridge have been removed to reduce congestion.
A dedicated hotline is to be set up to help commuters while the bridge remains closed for three weeks.
Secretary of State for Scotland David Mundell said: "The closure of the Forth Road Bridge until the new year is a very serious development which will have a major impact not just on local communities and travellers but on the wider Scottish economy.
"I met today with Fife Council and we discussed the impact the closure will have on the local area.
"While this is a devolved responsibility of the Scottish government and they now have the task of ensuring that all possible measures are taken to minimise the disruption during the bridge closure, the UK government stands ready to assist in any practical way we can."
The transport minister said he anticipated the 51-year-old bridge, which is due to be replaced by a new crossing in about a year, would be open for commuters returning to work in January.
Mr Mackay said there would be about a week of further inspections and two weeks to carry out the repairs.
He said the work was weather-dependent and it was difficult to predict exactly how long it would take.
The minister said additional rail, ferry and park-and-ride facilities are to be put in place.
Emergency service vehicles will still be able to use the bridge when responding to 999 calls.
Mr Mackay said "We are aware of the potential economic impact, for strategic traffic in the east of Scotland and on people living in local communities.
"This is an unprecedented challenge in the maintenance of the Forth Road Bridge. On balance following advice from engineers and independent experts, the full closure is essential for the safety of the travelling public and to prevent further damage to the structure of the bridge."
While ministers anticipate reopening the bridge to traffic, one expert cautioned there was a possibility only public transport would be allowed on the bridge in future.
Mack West, chairman of the Institution of Civil Engineers (Ice), told BBC Radio Scotland's Newsdrive programme: "I think there's a possibility that there will be restrictions on the existing bridge and it's likely that it may be restricted to public transport usage and cycling."
Rob Shorthouse, ScotRail communications director, said they had managed "so far" to increase the train capacity between Glenrothes and Edinburgh by about 25%, from 81 journeys a day to over 100 to help relieve congestion.
He said: "This is a really difficult set of circumstances because services are busy.
"All the trains that we have got are out there on the network.
"So, because this is such a national priority we are looking right across Scotland to see where we can move some capacity."
A Stagecoach spokesman said: "Stagecoach has on Friday operated a number of extra buses between Fife and Edinburgh and will continue to do so, wherever possible, to provide extra capacity to enable customers to travel by public transport rather than driving, helping to alleviate traffic congestion in the affected areas.
"Customers are able to use both Ferrytoll Park and Ride and Halbeath Park and Ride sites in Fife to leave their cars and take the bus to Edinburgh."
An NHS Fife spokesman said: "Any potential effect on patients and staff is currently being assessed and we are exploring the provision of accommodation and transport, where necessary, to ensure we continue to provide high quality care."
A spokesman for the Scottish Ambulance Service said: "Ambulances will continue to travel across the Forth Road Bridge when responding to emergencies.
"Arrangements are being made to maintain ambulance transport of non-emergency patients to appointments using alternative routes and hospitals, as appropriate."
He added that local teams were working with partners at NHS Fife, Tayside, Lothian and Forth Valley to mitigate any potential disruption to services for patients.
The southbound carriageway of the bridge has been shut since the discovery of steelwork defects during inspections on Tuesday.
Since then engineers have identified eight other parts of the bridge where similar problems could be encountered.
The decision to close the bridge completely was taken late on Thursday after the latest reports indicated the faults were more serious than had earlier been thought.
Traffic volumes have increase dramatically since the bridge's opening in 1964, shortening its projected lifespan.
Concern about corrosion of the main suspension cables, first confirmed more than a decade ago, prompted ministers to order a new crossing.
A new £1.4bn Queensferry Crossing over the Firth of Forth is due to open in December next year.