THE bomb squad was called in after workmen on a slip road leading up to the new Forth crossing spotted exposed wires.
Experts feared the discovery from an old demolition site dating back to the 1960s was potentially explosive.
Police announced that both the road and rail bridges were to be closed for two hours while a controlled blast was carried out.
Motorists heading into the Capital were sent on a 30-mile diversion via the Kincardine Bridge as commuters were urged to avoid the route during rush hour.
The bomb squad moved in just after 5pm as long queues built up on the A90 heading out of the city and trains were delayed and cancelled.
Police eventually reopened both bridges around 6pm and said there had never been any threat to the public.
By then, large crowds of frustrated passengers had gathered at Haymarket and Waverley as they waited for ScotRail services to resume.
Tax assistant Emma Doig, 19, was trying to get home to Dunfermline and was left waiting for more than an hour to board a train.
She said: “I left work at about 4.30pm because I heard about the disruption. At first I was really stressed but then I thought ‘There is no point worrying’.”
The wires initially caused the closure of the slip road from the A90 northbound to Ferrytoll interchange at lunchtime.
Police initially announced that the bridge would be closed between 4pm and 6pm, before stating that the explosion would be postponed until 6pm. It took place around 5.15pm.
Inspector Andy Amour, of the roads policing unit, said: “They were believed to be explosives of a historic nature, which had been there for a while. The experts came in and the advice was the best and safest action was a controlled explosion. In the interests of safety, we shut the Forth Road Bridge.”
He apologised for the changing timescales which were given out to the public, describing the incident as a “developing situation”.
Insp Amour said: “We were acting on the advice of the experts there. That’s why action was taken to shut the bridge when it was. The weather conditions were against us, the daylight was going so we had time limits. The road itself was shut for 45 to 50 minutes.”