Sydney Marathon the country's biggest ever

The Sydney marathon has boasted a record number of participants with thousands turning out to pound the pavement on a warm spring morning.

More than 17,000 runners registered to tackle Sunday's gruelling 42km course.

The figure was more than double the number who took part in Australia's previous biggest marathon, held in Melbourne in 2019 with 8100 runners.

The event was vying for inclusion as an Abbott World Marathon Major, alongside iconic races in New York, Boston, Chicago, London, Tokyo and Berlin.

Last year, Sydney became the first marathon in the southern hemisphere to be given Platinum Label status by World Athletics, and it is now in its second year of a three-year candidacy period to become a major marathon.

Tourism Minister John Graham said Sydney had a long history of hosting major international sporting events.

"No city in the world has a greater combination of natural beauty and iconic infrastructure for hosting a marathon than Sydney," he said.

"With so many incredible vantage points and entertainment hubs added to the event, the Sydney Marathon is going to provide a festival atmosphere for spectators and runners."

Runners were met with warm temperatures on Sunday morning with Sydney a comfortable 20C about 7am.

The temperature rose later in the morning, with the city expected to reach a top of 30C on Sunday.

NSW Police warned Sydney would be busy because of the marathon and reminded people to adhere to heat messages, keep their cool and look out for each other.

The Bureau of Meteorology was predicting heatwave temperatures for the NSW South Coast in coming days.

NSW Police had not recorded any major incidents at the marathon as of about 9.30am on Sunday.

The event included five races - a half and full marathon, a wheelchair marathon, a 10km bridge run and a family fun run.

To help raise the marathon's profile, four spectator sites were set up in Pyrmont, The Rocks, Surry Hills and Moore Park.

The sites offered free coffee, food trucks, custom sign workshops, DJs, drag queen performances, prize giveaways and large screens broadcasting the marathon live.

The Sydney Harbour Bridge, Cahill Expressway and Western Distributor were to be closed from 4am to 10am on Sunday.

The Sydney Harbour Tunnel will remain open but extensive delays are expected.

Australian Associated Press