About 50 students from the city's Military Preparation College picked up litter and gathered rubbish to form a huge pile of rubbish.
The group did not stop until every piece of litter from the hill had been picked up during the two-day operation.
Richard Jones, a warden for Portsdown Hill Countryside, said: 'It is very unusual to have that many people helping out and we are very grateful to them.
'They came along and took one look at the litter and just got on with it. They even went into the scrub to remove all the unsightly rubbish that was embedded in the land and had been for many years.
'It is difficult to entice volunteers to plunge into a scrub so it is great that a group of youngsters took up the challenge.'
He added: 'The land had grown around the rubbish and we have wanted to clear it for ages.
'Now the pile of rubbish will be taken away and disposed of.'
The litter pickers were amazed at what they found while they were carrying out their task.
James Dean, 17, said: 'We picked up a car engine, a broken TV, loads of broken glass bottles, beer cans and packets of crisps – all sorts of things.'
The students who attend the college in Twyford Avenue, North End, feel this work is vital for military preparation and will gain a qualification in community work for their efforts.
James added: 'I want to go into the Army and it is all about helping in the community.'
Katy Burnage, 18, said: 'It was hard work taking all the rubbish from Portsdown but fun.
'It was good to give something back to the community and it felt great when it was finished.'
Military Preparation College tutor Brian Richardson said he was impressed that the enthusiastic students chose to return for a second day.
He said: 'I am very pleased to see students getting involved in this kind of work – it has such an impact on the community. They were actively involved in helping the environment.
'Portsdown Hill has been stripped back to its natural beauty and people can now enjoy it.'