A runner was seen breaking down in tears after crossing the finish line at this year's Nanchang International Marathon in the capital of east China's Jiangxi Province, revealing the touching story behind his motivation to race.

The runner is a 40-year-old father named Li Gang and he was seen clutching a photo of his teenage son. After crossing the finish line, Li collapsed to the ground crying and paramedics rushed over to check on him.

Weeping in sadness, Li told reporters,

My son should have been here with me. We ran the marathon together for the first time last year. We finished it in about 3 hours and 40 minutes. My son told me then: ‘It's okay, dad, we'll run again next year’.

It's sad. But this runner did fulfil his promise. 😥 #RunSociety #Spacebib
Read more: https://t.co/acLvMbPQ7P pic.twitter.com/U5m1OFF1Gp

— RunSociety (@Run_Society) November 15, 2018

Tragically, Gang’s 16-year-old son could not fulfil the promise as he has drowned in an accident this August during a summer holiday, shortly after the pair had signed up for the race.

The grieving father, Li Gang, decided to pay tribute to his son by bringing him on the emotional 42-kilometre journey last Sunday.

According to a local news publication, 40-year-old Gang crossed the line in 02:55:52, coming 34th in the men’s race. His 16-year-old son was his entire motivation to keep going. Whenever he felt like giving up, he would look at his son's photo, which he had kept in his glove during the entire race.

'I promised him I'll be here,' he told reporters. 'I promised him I'll finish the race under three hours.'

'Daddy made it! Son, did you see?' he said.

Nearly 30,000 runners from around the world took part in the event which consisted of a full marathon, half marathon, mini run and a family run.