Hundreds of people applying to join the police have gone on the rampage in the northern Indian city of Ghaziabad.
Riots began because many felt a written test was too difficult, officials say.
The mob of would-be officers rampaged along a stretch of the main road to Delhi, attacking people and property. Almost 30 people were arrested.
Passengers were forced from their vehicles and reports say several women were molested. It took police an hour to bring the riots under control.
About 20,000 hopefuls had turned up on Sunday evening to take the written test assessing their suitability to be constables in the northern state of Uttar Pradesh police.
The state's home secretary, RM Srivastava, told the BBC that the candidates were upset because they found the question paper too difficult.
As they left the police headquarters in Ghaziabad, several hundred angry applicants began stopping cars and buses on the highway, breaking windows and demanding lifts.
The mob then ran out of control along a 5km (three-mile) section of the main highway to Delhi, smashing fences, looting food from kiosks and beating up shopkeepers.
Security forces baton-charged the rioters to restore order.
Uttar Pradesh police say they have now obtained still photographs and film of the riots and are busy identifying those responsible so their applications to join the police can be rejected.
India's police force is battling an image problem
"These men will become gangsters if they do not become policemen," one man in Ghaziabad was quoted as saying by the Times of India newspaper.
The unrest comes as the government is trying to reform the police services.
Complaints of police corruption and heavy-handedness are commonplace in India.
The BBC's Damian Grammaticas in Delhi says most Indians have little faith in their police, viewing them as lazy, corrupt and inefficient.
The riots will do little to change that perception, our correspondent says.