Two people — including a 16-year-old boy — were struck by bullets in East Harlem Tuesday afternoon in what police said was a burst of gang-related gun violence tied to the earlier shooting of a student on the West Side.

The teen and four other students at Harlem Renaissance High School were going out for lunch at around 1 p.m. when three men came up to them and a fight broke out between the two groups at East 129th Street and Madison Avenue, police officials said.

Bullets flew during the brawl, leaving the teen — who sources said was on his first day at the school — with a bullet wound to the leg. A 27-year-old man — an innocent bystander — was also shot in the leg and rushed to Harlem Hospital, according to cops.

The three suspected shooters fled, cops said. Police believe the teen was the intended target.

Police sources said the violence could be a retaliatory strike stemming from an earlier shooting that locked down two Big Apple high schools on the Upper West Side just three hours earlier.

A third incident — involving a report of shots fired at East 105th Street and Park Avenue at around 3 p.m. — was also believed to be related, police said.

“The last five hours we got three shooting incidents in the northern side of Manhattan,” NYPD Chief of Patrol John Chell said at a press briefing following the East Harlem shooting.

“Right now, we’re proceeding like they’re all connected,” he added. “Why do I say that? In proximity, geography, around schools, age of our victims, and now we confirmed at least one incident, this incident here is gang-motivated.”

A witness to the shooting that left the 16-year-old injured said she “saw two of his friends carrying him across the street” and that “He was limping. He was in pain.”

“I said, ‘No, no, no,'” she recalled. “You gotta wait. You don’t know what kind of bullet it is. It could be traveling through his body.”         

“There were people out there, there were kids out there,” she said.      

The woman, who identified herself only as Candace, said the older victim, who was hit in the right calf, sat on a nearby stoop.

“The blood was gushing and gushing,” she said. 

Another witness told The Post: “It seemed like it was planned.”

“They were waiting for him,” he said.

The NYPD was mobilizing to curb the violence, with cops “deploying to areas the best we know to slow this down,” Chell said.

“We’ll pull multiple resources … We’re gonna flood this area,” the chief said, adding that cops in the NYPD Patrol Borough Manhattan North on the day shift will be held over to join officers being brought in from other precincts and boroughs — with the show of force continuing through the night and up until school dismissal on Wednesday.

In the earlier Upper West Side shooting, a 17-year-old high schooler was shot at least three times during a fight with other teenagers at Amsterdam Avenue and West 68th Street at around 9:50 a.m. — with the alleged 19-year-old shooter later busted.

That shooting forced two nearby schools — The Maxine Greene High School for Imaginative Inquiry, which the victim attends, and LaGuardia High School of Music — to go into lockdown.

The wounded teen — a “chill” teen nicknamed “Nas,” according to one classmate — was rushed to the hospital and was in stable condition after undergoing surgery, according to police.

The two shootings in which teens were injured and the third incident — the report of shots fired where four shell casings were recovered — are all believed to be connected.

“We’re proceeding as if this is all connected,” Chell reiterated.

Additional reporting by Larry Celona