Customers and staff at the Welcome Inn in Well Hall Road, Eltham, south-east London, were treated for eye, nose and bronchial problems after the apparently unprovoked attack.
Scotland Yard said two men in their 20s let off canisters of what was believed to be CS gas or Mace after drinking in the pub.
'Ambulances were called but police stopped an empty double-decker bus that was passing and commandered it to take the victims to Brook hospital, Woolwich,' the Yard said.
The attack in the Beefeater house took place at 8.30pm. The pub's manageress, Margaret ChangChao, said: 'The men just came into the bar and started spraying the stuff around.
'They sprayed it at my assistant and he was bent over double. I thought he was messing around at first and then it hit me in the throat.
'We called the appropriate services but the policeman told me he had managed to flag down a bus to take everyone to hospital and cancelled the ambulance.
'I want to know why they did it. It was absolutely ludicrous. I was shocked but at the time I thought: 'I can't panic. I've got all these people in here'.
'But now I think 'God, what have they done?' I can't believe that young people behave like that these days.
'I think it may have something to do with the fact that I have had to bar a few young people from the pub - they want to drink the strongest stuff behind the bar and take on the world.'
Scotland Yard said there was no known motive for the attack.