Alarmed victims who were in Uxbridge town centre were left in shock after moped riders sprayed them with fire extinguishers in three separate incidents.
Youths riding on the bikes struck in York Road, outside the tube station and Uxbridge High Street between 10pm and 11pm on Boxing Day.
Getwestlondon spoke to two of the innocent bystanders who were startled after coming across the youngsters.
Augusta Overton said she and four others were walking down the High Street when they saw bright lights coming towards them.
It became clearer that it was three youngsters on two mopeds as they swerved near them, according to Ms Overton.
She said: "They missed us by about half a metre and sprayed all five of us with the fire extinguisher.
"It got in our eyes, in our mouths and on our clothes, we couldn't see a thing."
The 22-year-old from Aylesbury described the horror of seeing the riders return as they tried to get to their car in York Road.
"The same mopeds drove passed and did the exact same thing," she said.
"Fortunately we are okay but we are concerned - what if there had been a child or an elderly person or something with health issues?"
Rider said 'sorry mate' before attacking with extinguisher
In another incident, 44-year-old Marcus Man left the tube station's bus stop exit.
He said: "There was already a bike there but as I was going to get the bus, a second bike came and the rider said 'sorry mate' and then there was this powdery substance eveywhere.
"The guy in front of me was covered and I got some of it on my clothes and glasses."
The Uxbridge resident continued: "We didn't know what it was and called the police - it was all over in about 20 seconds but it was shocking, especially when you don't know what it is."
After being helped by a commuter who was their "good samaritan" and gave them tissues, another lady with two children told Mr Man she was almost attacked, but after spotting their two children, they backed away.
Police told victims that although the spray was not poisonous, it could result in a check up for those with respiratory conditions.
A Metropolitan Police spokesman told getwestlondon: "We did get antisocial behaviour calls between 10pm to 11pm, but thankfully no one was seriously hurt.
"No arrests have been made regarding the three incidents."
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