The Forth Road Bridge is to remain shut until the new year after faults were discovered in its steel work, Scotland's Transport Minister Derek Mackay has said.
The decision to close the bridge was taken by the Scottish Government after inspections carried out by specialist engineers and following advice and assessment of the fault by independent experts.
Work is already under way to repair the crossing and it is expected to be reopened to traffic in January.
The complete closure of the bridge came into force at midnight, with major tailbacks experienced on diversion routes at rush hour.
Mr MacKay said the decision was "not taken lightly", and steps are being taken to lessen the impact of the closure.
The problem was first identified during a routine inspection on Tuesday. Traffic was restricted that evening but it was later decided that the bridge should be closed entirely.
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 calls.
Engineers said a 20mm crack in a truss under the southbound carriageway close to the bridge's north tower could not have been predicted and happened quickly.
Continuing to allow traffic to use the bridge would "increase the risk of causing extensive secondary damage to the structure".
Mr Mackay said: "Every effort is being made to open the bridge as quickly as possible but safety is the main priority, however these works are weather dependent given the height and location of the bridge.
"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.
"The bridge operators Amey have a robust inspection team in place and these defects are problems that have only occurred in the last number of weeks.
"We are taking every step we can to lessen the impact of this closure. Action now will mean that any closure is much shorter than it might be if we waited."
Chartered engineer Mark Arndt, from Amey, said: "This is a complex engineering challenge. The component failure is in a difficult-to-access location and our response is also highly dependent on weather conditions.
"We continue to work around the clock on inspections, assessments and calculations along with the development of designs to effect the necessary repairs, while at the same time mobilising all the resources required to reopen the bridge as soon as is possible."
Engineers said further damage to the section could have caused the bridge deck to drop by around six inches and would have led to the closure of the crossing for "a number of months".
About 70,000 vehicles cross the bridge each week day, and around 24 million every year.
Long delays were reported on alternative routes during Friday morning's rush-hour, with Traffic Scotland warning of tailbacks stretching to 11 miles at one point on the approach to the Kincardine bridge, while there was a six-mile queue around the Clackmannanshire bridge.
ScotRail said it is adding extra carriages and staff for services on the rail bridge to and from Fife, saying it is a "national priority".