An armed police officer who was “deliberately driven at” is in hospital with serious head injuries.
Officers were called at 3:30 BST to an assault in Belvoir Street and two men, aged 19, and two men, aged 20, from Leicester, were arrested.
Minutes later, an officer carrying out inquiries in the case was injured in the Bradbourne Road area of the city.
Police said “early inquiries suggest the officer was deliberately driven at by a car which then left the scene”.
The officer’s injuries are not believed to be life-threatening, Leicestershire Police added.
Officers said they were currently linking the two incidents in Belvoir Street and Bradbourne Road.
Deputy Chief Constable Rob Nixon tweeted that there was a “full-scale operation” running to find those responsible.
Mr Nixon also said on Twitter he had spoken to the injured armed officer who was “receiving good medical care”.
He added he was “proud of the efforts being put in by all teams to locate and arrest those responsible”.
Police and the public have been wishing the officer well on Twitter.
“This is a serious incident which saw one man injured and one officer seriously injured while he was on duty,” Mr Nixon said.
“This is completely unacceptable and will not be tolerated. We are fully investigating this incident and a team of officers and staff are carrying out full inquiries.
“If you have any information in relation to these incidents, we urge you to make contact with us.”
Matt Robinson, secretary of Leicestershire Police Federation, said: “It’s always difficult to hear news of a colleague getting injured whilst on duty especially in these circumstances.
“Deliberately driving a vehicle at an officer is nothing more than an attempt to take a life.”
It “brings into focus the risks police officers are expected to take” to keep communities safe, he added.
Police said the man assaulted in Belvoir Street, who is aged in his 20s, did not attend hospital for treatment and the four men arrested in connection with this are in custody.