Passengers had to evacuate a bus when it filled with smoke.
A seat was set alight on the number five service in Sackville Road, Hove, last night.
Firefighters were called to deal with the blaze shortly after 8pm.
No serious damage was caused but the bus had to be taken out of service.
Roger French, managing director of the Brighton and Hove Bus and Coach Company, said: "This was despicable. I hope the police catch whoever was responsible."
Inspector Mark Powells said: "We have taken CCTV footage from the bus to try to identify the offender."
Anyone with information is asked to call Brighton and Hove police on 0845 6070999.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article