An Adelaide father has been hospitalised but is being hailed a hero after shielding his children from a giant falling tree at a birthday party in a popular suburban park.

Key points: Several children were playing under the tree when it cracked and fell

Several children were playing under the tree when it cracked and fell A grandfather and several parents rushed to protect them

A grandfather and several parents rushed to protect them A man in his 30s is in hospital with back injuries

The accident happened at Tusmore in Adelaide's east where friends and family had gathered to celebrate a girl's third birthday party.

Several children were playing under the tree at the park on Barr Smith Street when it gave way before 12:30pm, causing terror among the picnic-goers.

"We heard a crack and then part of the tree fell down," witness Bianca Di Brino said.

"The right side came down first, probably about 10 minutes later the left side came.

"A few people actually got stuck underneath it. One of them has actually gone to hospital … he got hit in the back protecting his children."

The man, who is aged in his 30s, was taken to the Royal Adelaide Hospital with back injuries, but is in a stable condition.

Several children were playing underneath the tree when it fell. ( ABC News: Samantha Dawes )

The birthday girl's grandfather, who is aged in his 70s, rushed over to the tree to provide help but was also taken to hospital with chest pain.

Two children were treated at the scene for shock, but no-one sustained life-threatening injuries.

"There were actually five or six underneath the tree when it collapsed so thank God … whoever was around just grabbed whatever child they could," Ms Di Brino said.

"You kind of go into crisis mode, you kind of just gather the children, making sure everything's OK around you. But I guess just shock and fear.

"It's a weird feeling."