It happened around 8.15am on Clatterbury Lane in Clavering - a village between Bishop's Stortford and Saffron Walden.

The ambulance service said two children were taken to hospital in Cambridge but are believed to only have minor injuries.

An air ambulance and fire crews were among the vehicles sent to the scene.

Essex Fire and Rescue Service said the crash on the narrow lane involved a bus and a car.

"The bus had also collided with a tree branch causing extensive damage to the roof," it added.

"Nobody was trapped in either vehicle and firefighters worked alongside partners to make the scene safe by 9am."

A father who arrived to give first aid described it as "carnage" and "pretty horrific".

"It could have been really major," said Martin Philpott - who had two children on the bus.

He told Sky News: "[The children were] very, very lucky they got away with what they did, little cuts, bruises. But it's a miracle really."

Mr Philpott said it appeared the bus hit the tree while passing the car, with the roof landing on another car belonging to a woman whose son had just got on the bus.

He said one pupil seemingly had a lucky escape after ducking just before impact.

"One of the boys at the front - he was at the front on the side where the tree was - apparently dropped his phone, he leaned down to pick up his phone and that's when it hit..."

Pupils from two schools - Saffron Walden County High School and Joyce Frankland Academy - were on the bus.

The academy said the crash involved the number 446 Stephenson's school bus and that it would have been "traumatic" for pupils.

It said some students were having cuts and grazes looked at, but that no serious injuries had been reported.

A statement added: "Parents of the students involved are being contacted directly and the school has made arrangements to collect the students and offer a space inside the school for anyone who is in shock or upset by the events."

Sky correspondent Rachael Venables said the road was still closed at Thursday lunchtime but that a tow truck had arrived to remove the bus.