The 31-year-old competitor finished the 15-kilometre race in a time of 44 minutes and nine seconds, despite fighting a bad bout of hay fever in the lead up to the event.

Dejen Gebreselassie came in second place just one second later after leading for most of the fun run. Jack Rayner was third.

The first woman to cross the line was Julie Norney, of Kilsyth, at 56 minutes and 58 seconds.

Adams said he was thrilled to win the event, despite having to cut back on training due to breathing difficulties.

"He pushed me right to the line," the Moonee Ponds runner and Commonwealth Games hopeful said of the second-place finisher.

"I didn't feel that good out there from my hay fever," he added.

"This year, I don't remember it being as bad as it was. But it's all right. It's great to get the win out."

City2Sea, presented by Westpac, is an annual event that includes a 15-kilometre and five-kilometre inner-city course from the MCG to St Kilda's Catani Gardens.

Competitors can also complete a 15-kilometre relay course.

Its sister event, the Sun-Herald City2Surf, is the biggest fun run in the world.

Norney was all smiles when she saw her mother, sister-in-law, niece and nephew were cheering her on from the sidelines as she finished the 15-kilometre race in top form.

"I didn't know they were coming, so it was nice to see them," the 49-year-old said.

She didn't expect to win the female event either. Erchana Murray-Bartlett came in second, at 37 seconds later, while Ciara Boyd-Squires Long was third.

"It was a good opportunity for me as I haven't been running and it was a bit of a kickstart," she said.

"You never know as the wheels can fall off and the girls are coming right behind you. So, it was a huge surprise, a very pleasant surprise."

In the men's race, Gebreselassie had set the pace fast in his first attempt at the City2Sea, but fell just a stride or two behind Adams for a thrilling finish.

"I wanted to make it a hard race with a fast time," he said after getting his second-place medal.

"But Liam [Adams] is really strong. He's an Olympian runner. I have a lot of respect for him.

"He helped me out as well. He was right on my toes. It was good to have. I'm happy. It was a tough race."

Competitors set off at 8.15am on Sunday from the MCG under foggy, overcast conditions, but many finished the race under sunny skies.

Scott Clark, 46, was among them, donning his "Green Warrior" mask and cape for his first attempt at the City2Sea.

"It's a fantastic race," he said. "I run just for fun, to keep fit, and to meet great people. I've made a lot of good friends through running."

The popular event is not just for running enthusiasts, but for anyone who wants to raise money for charity.

Wendy Jenkins, a board member of the Lungitude Foundation, said the health charity managed to put together a group of about 60 people: patients, staff, researchers and carers.

"It's a great event. I think we're going to do it next year as well," she said.

Organisers said more than 8000 racegoers competed to raise more than $240,000 for hundreds of charities.

One of the biggest groups was a 50-strong team donning bright yellow shirts, who were raising funds for CFC ANCOP, a faith-based charity aimed at supporting educational programs in the Philippines, Papua New Guinea and the Solomon Islands.

Sophia Banzon, 23, said competitors don't have to be athletes to join in on the fun.

"We didn't really train - we just tried our best," she said, laughing.

"We just really wanted to participate and make ANCOP known to more people."

One of her running colleagues, Joe Parayno, 35, agreed.

"It's so hard to wake up early on Sunday morning," he said, "but it was actually worth it because you're doing something for a good cause."