Dramatic photos captured the aftermath of what is believed to have been an acid attack near Chafford Hundred train station last night.
The incident took place shortly after 8pm and all three emergency services rushed to the scene.
Witnesses described victims being hosed down by firefighters as police swarmed the area.
One witness estimated about 15 emergency vehicles on scene while young men could be seen being treated with water and foil blankets.
A spokesman for Essex Police said: "We are investigating an incident in which four people were attacked with an unknown substance in Grays.
"We were called shortly before 8.10pm on Monday 25 February with reports four men had been assaulted by a group of men on a footbridge linking Chafford Hundred train station and intu Lakeside shopping centre.
"They have been taken to hospital with facial injuries, which are not believed to be life changing or life-threatening.
"A 17-year-old boy was arrested on suspicion of causing grievous bodily harm. He has also been taken to hospital as a precaution.
"Enquiries are ongoing to find two other people believed to have been involved in the disturbance.
"We are aware there were witnesses at the scene who filmed the incident and we ask them and anyone with information to call Grays CID on 101 quoting incident 1136 of February 25.
"Alternatively, contact the Crimestoppers charity 100 per cent anonymously on 0800 555 111 or use the anonymous online form at Crimestoppers-uk.org"
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