Bus carrying wedding guests catches fire on Sydney Harbour Bridge

A 50-seater bus carrying a pair of newlyweds and their bridal party has burst into flames on the Sydney Harbour Bridge overnight.

The bus was travelling into the city along the Bridge about 11pm when the vehicle caught fire.

A taxi driver tried to flag down the bus after he noticed a flame underneath it but it was too late. 

The bus driver tried to use water and a fire extinguisher to quash the flames but they spread quickly and emergency crews were called.

Thankfully, all 45 passengers managed to escape without injury.

"It was a quite ferocious fire in terms of the fire. The bus was fully involved in fire with flames over two storeys high," one witness told 9NEWS.

The fire at one point threatened to spread to a nearby RMS toll booth.

The smoke was so intense it set off fire alarms in neighbouring buildings.

Wedding guests are now demanding answers as to why the bus caught flames on the special day. 

The bus company says it spent thousands of dollars getting the vehicle serviced recently and that it had only been back on the roads for a day before it caught fire.

Police are now investigating exactly what went wrong.