A number of pupils from one of the city's schools were taking part in a group chat via social media earlier this week when an unknown man joined in.
He went on to expose himself before performing a sex act during the incident on on the live video streaming service.
It prompted a warning to parents in the city yesterday, with at least one school issuing a letter detailing the incident and urging parents to delete the live.ly app from their children's phones.
Today a police spokesman said: "Specialist detectives from Leeds District Safeguarding Unit are investigating and enquiries are ongoing to identify the person responsible.
"Officers are also liaising with Leeds City Council and working with schools to offer support to pupils and to reinforce messages to pupils and parents around online safety."
The letter issued to parents said that Monday's incident was not the first involving the live.ly video streaming platform to be reported.
It said Leeds safeguarding team had received at least one other call in the past week voicing similar concerns about the app, which is linked to the musical.ly platform.
Earlier this month schools in Liverpool were also reported to have issued letters detailing concerns about the musical.ly app.
Liverpool Cyber Safety Team had warned the app was increasingly being used by those wishing to groom children, due to the age of most of its users.
The letter about live.ly issued in Leeds explained: "Whilst the app is only supposed to access contacts on the user's phone, it appears that quite quickly, friends of friends of friends are able to join the group chat whom your child does not know at all.
"It is therefore an unsafe app for children to use, as it risks either an incident such as happened, or opportunities for your child to be groomed online and therefore at risk of sexual exploitation."
Anyone with concerns about a specific incident should report it to police via 101.