South Road will be closed indefinitely between Anzac Highway and Cross Road after debris fell from the cycling and tram overpass this morning.

The major arterial road through Adelaide has been closed in both directions until further notice after reports of falling material.

Tram services have resumed between the city and Glenelg but are not stopping on the overpass.

Debris from the South Road overpass. ( ABC News: Malcolm Sutton )

Transport Minister Stephen Mullighan said closing part of such a key road would be a major inconvenience, but it was necessary.

"The problem with the bridge seems to be the outer most span of the bridge, which is supporting the walking and cycling path, appears to have shifted slightly off its bearings. There are two sections which appear to have been affected," he said.

"Obviously safety is our first concern when it comes to this issue.

"South Road will remain closed to traffic until further notice and we are encouraging motorists ... to avoid the area.

"We've closed off between Anzac Highway and Cross Road, and this is important so we are not getting people who are travelling down parts of South Road expecting to get under this overpass."

This afternoon Mr Mullighan tweeted work to repair the bridge would begin tomorrow.

He said between 35,000 and 45,000 people use South Road each day, and a proportion of the 10,000 daily trams users.

"We are talking tens of thousands South Australians who have been impacted by this and that's why we take this issue so seriously."

Engineers to examine overpass

A Department of Planning, Transport and Infrastructure spokesman said earlier today a structural engineer had been called to inspect the site.

A large section of concrete has moved from its bearings beneath the walking and cycling path. ( ABC News: Malcolm Sutton )

Adelaide cyclist Lew Toop said it looked like part of the South Road tram overpass could collapse onto the road.

He was stopped by other cyclists who spotted the damage.

"It was pretty obvious a main section of the walkway had tilted badly away from the tram platform," Mr Toop said.

"It was very visible and it looked like if it did collapse it would fall onto South Road and basically block the whole of the road."

Cracks visible under bridge

He said the underneath looked like a concrete support frame had "cracks going up".

A concrete section of the South Road overpass in Adelaide has moved. ( Supplied )

A department spokesman said the traffic management centre received reports of debris falling from the South Road tram overpass about 7:00am.

He said an independent bridge expert would also be consulted.

Tram services were disrupted for most of the morning and at 11:20am Adelaide Metro tweeted that trams had resumed but would not stop at stop six on South Road.

Trams had been running from Glenelg to stop eight at Beckman Street, and from the Entertainment Centre to South Terrace.

SA Police have asked motorists to avoid the area.

Traffic has built up on surrounding roads including Goodwood Road due to the closure of South Road. ( ABC News: Malcolm Sutton )

Bridge less than 10 years old

Mr Mullighan said the overpass was just seven to eight years old.

The bridge appears to have slumped beneath the walkway. ( ABC News: Malcolm Sutton )

McConnell Dowell were awarded the design and construction contract for the $30-million project by the State Government in April 2009.

The company received an award for excellence for the project at the Institute of Public Works Engineering Australia Excellence Awards in May the following year.

"All the necessary and appropriate actions were taken by the Government when we were building this piece of infrastructure," Mr Mullighan said.

"We're going back through all of that process, going back and talking with the people involved at that time, and going back through all of the paperwork, just to see what might have contributed."