New Delhi, Dec. 16 -- Hungarian Prime Minister Viktor Orban 's main challenger on Saturday led tens of thousands of demonstrators through the streets of Budapest in a protest over alleged child abuse in state-run juvenile institutions, an issue that has already shaken Orban's government following a scandal last year.

The demonstration, called by Orban's top rival, Peter Magyar, came in response to videos published this week depicting employees of a juvenile correction facility in Budapest physically abusing children housed there. The former head of the facility, who is in police custody on suspicion of operating a prostitution ring among other crimes, has also been accused of subjecting minors to physical and sexual abuse.

Police raided...