Council bosses are hunting prolific graffiti vandals who have daubed 2,500 tags across the city, costing thousands of pounds to clean up.
The authority has spent almost £30,000 scrubbing away the tags – Kase and SO 12 – from its buildings in the past few months.
It fears the bill could run to more than £200,000 by August.
Businesses have also paid thousands of pounds to clean them up since the tags first started appearing last summer.
The council has now taken the unprecedented step of offering a reward of £500 to anyone who can help secure convictions.
Assistant mayor Councillor Sarah Russell said: "These two tags are becoming so prolific it is becoming difficult for us and for businesses to bear the cost of constantly removing them.
"In the past, we have shied away from offering rewards for information. But given the scale and cost of this problem, we feel we have to try something new to encourage people to come forward with information.
"Hopefully, it will work and we will get the information we need to secure convictions.
"These tags should not be seen as an inevitable part of the city, they are ugly."
The council, which has mounted a number of unsuccessful surveillance operations to try to catch the taggers in the act, believes two gangs, rather than individuals, could be responsible.
The designs were both spotted for the first time in Saffron Lane in August, but have since cropped up throughout the city.
Adrian Russell, council director of environmental and enforcement services, said: "We've never had such prolific tagging before.
"We have had days where we've come in to work and overnight there have been anything between 60 and 100 reports of these tags appearing.
"It is very difficult to catch the people responsible.
"We really need help from the public if we are going to find these people and put them in front of the courts.
"Someone must know who they are and we want to hear from them."
London Road was hit in November. Manoj Keshavji, chairman of London Road Business Association, described it as an "onslaught".
Mr Keshavji said: "They appeared in a very short space of time.
"London Road is the gateway to the city and there are a lot of fine, historic buildings here.
"But these people have no respect for that and they need to be caught."
Mr Keshavji and other traders worked with the city council's wardens to remove the SO 12 tag. It has not reappeared.
A Saffron Lane shopkeeper, who asked not to be named, said: "These things are ugly and expensive and difficult to remove.
"The people who are doing it need to be caught. If I had my way the courts would get them to remove them one by one and then put them in prison."
Anyone with information about the people responsible for either of the tags should contact Leicester City Council's city wardens team on 0116 252 6430.
All calls can be treated as confidential.
Names and other details can also be e-mailed to:
city.wardens@leicester.gov.uk
