Firefighters were called to the incident at the recently-replaced wooden bridge at about 3.25pm.
Andy Walford, acting crew manager, said: ‘A drunk male decided to throw his barbecue ashes across the bridge so no-one could get in or out.
‘As we turned up they were putting water on it.
‘We spoke to the male and gave him a talking to because it’s very dangerous.
‘People couldn’t get in or out and there were kids in there, luckily it was raining.
‘He was drunk, he was 17 and thought it would be funny trying to impress his mates.
‘It was wet, luckily it didn’t catch on fire but there was a couple of scalding marks and the bridge had only just been replaced.
‘We called the police, there were there as well.
‘We all gave him a swift talking to.
‘They’re going to speak to his parents.’
The small fires vehicle attended from Southsea station but left at 4pm.
Hampshire police were called out and a spokesman said that as nobody had a made a criminal complaint, no further action was taken.