A truck driver has been arrested after two teenagers were struck on a pedestrian crossing outside an Adelaide high school on Wednesday morning, critically injuring one 16-year-old and seriously injuring another.

The girl and boy were hit by a truck while crossing Kensington Road outside Marryatville High School, in Adelaide’s east, just before 8am.

The students were rushed to the Royal Adelaide Hospital for treatment, while the 60-year-old driver was not injured.

South Australia Police said later on Wednesday he had been arrested and charged with two counts of causing injury by dangerous driving.

He will appear in the Adelaide Magistrates Court on June 7.

Earlier, South Australia Police superintendent Narelle Kameniar said the teenagers were hit while crossing at pedestrian lights.

“A truck delivering goods was involved in a collision with these two students,” she said.

Superintendent Kameniar said the 16-year-old boy was in a critical condition while the girl had serious injuries.

She said the “extremely traumatic incident” had happened in peak-hour traffic. It was witnessed by many other students, who were left distressed and upset.

Superintendent Kameniar said police were obtaining “numerous witness statements”.

A senior staff member at Marryatville High said the school had offered support to students.

“We understand that events such as this can cause distress for some people. We have additional counsellors at school today to provide wellbeing support to anyone impacted,” a message stated.

One tearful witness spoke of his horror.

“For the parents, and grandparents, it’s tragic, simply tragic,” he told the Nine Network.

Kensington Road remained closed to all traffic between Portrush Road and Hackett Avenue for several hours. It had reopened by mid-afternoon.

The Marryatville incidenct came just hours after a cyclist was critically injured in an accident at Darlington, in Adelaide’s south.

Police said the cyclist crashed on Flagstaff Road at 6.40am on Wednesday and no other vehicles were involved.

The 57-year-old male rider was rushed to the Flinders Medical Centre in a critical condition.

Flagstaff Road was closed to all city-bound traffic from Happy Valley Drive and Black Road while major crash investigators examined the scene.

-with AAP