Police are seeking three boys aged 12 to 15 after two men were seriously injured in a homophobic knife attack in Liverpool.

The men, both in their 30s, were walking down Manningham Road in the Anfield area of the city at 9.20pm on Saturday when they were approached by three male youths, Merseyside police said.

The boys made homophobic insults before one of them produced a knife and assaulted the men.

One of the men sustained injuries to his head and neck described as serious but not life-threatening, while the other sustained a minor hand injury.

The assault is the latest in a series of homophobic attacks involving teenagers. Earlier this month, five people aged 15 to 18 were arrested after punching a woman and her girlfriend on a London bus when they “refused to kiss” on demand for the boys.

A week later, two women were hit by an object thrown out of a passing car in Southampton while kissing in the street. Lucy Jane Parkinson and Rebecca Banatvala, both actors, were appearing in the play Rotterdam, which tells the story of a young gay woman, at Southampton’s NST Campus, when they were attacked. Parkinson said they heard “young boys laughing” as the car drove off.

The number of homophobic and transphobic hate crimes, including stalking, harassment and violent assault, have more than doubled in England and Wales over five years, Guardian analysis has shown.

Between 2014 and 2017, more than 2,500 people under 19 were arrested on suspicion of homophobic assaults, according to police data.

DI Tara Denn of Merseyside police said of the Liverpool incident: “This was an appalling and unprovoked attack on two men simply making their way home and we are working tirelessly to locate those responsible.

“Two men have been left with significant injuries tonight and the hate and violence that has been inflicted on them is simply unacceptable and won’t be tolerated on the streets of Merseyside.”

Denn appealed for witnesses and dashcam footage of the attack, saying one boy was wearing a dark “bubble” coat and another was riding a bike.

She added: “There is absolutely no place for hate crime in society and we know how much of a massive impact it has on victims. I want to reach out to the LGBT community, who I know will be deeply saddened by this news, and reassure you that we stand with you and if you report such incidents to the police, we will do our utmost to support and protect you and bring those responsible to justice.”

The UK Pride Organisers Network, which brings together organisers of more than 150 UK LGBT+ Pride events, called for tougher punishments for those responsible. It tweeted: “With a huge increase in reported hate crime and significant rise in online hate we must stand together and say NO! @ 10DowningStreet Please act now! Let’s get those responsible locked up and with harder sanctions!”

Laura Russell, the director of campaigns, policy and research at the charity Stonewall, said: “We’re still not living in a society where every LGBT person is free to be themselves and live without fear of discrimination and abuse. This attack is the third serious incident to make headlines this Pride month and another upsetting reminder of how much we still have to do for LGBT equality.

“From our research into hate crime, we know that four in five anti-LGBT hate crimes go unreported, with younger people particularly reluctant to go to the police. That’s why we’re working with police services across Britain and the Crown Prosecution Service to better support LGBT people reporting abuse.”

Earlier this month, Ryanair was criticised after asking a customer to fill in a complaint form after she said flight attendants had failed to intervene when she was called a “dyke” and “lesbo” by a gang of men on a flight to Seville, Spain.

Laura Muldoon posted photographs on Twitter of the alleged perpetrators giving her the finger, writing: “First holiday snap! Of this bunch of lads who chanted that I was a “miserable bitch”, “dyke” and “lesbo” (very well observed!) on @ Ryanair flight 12.27 from Stansted to Seville today. Oh yeah, flight crew did nothing.”

Those criticising Ryanair said the seat numbers were clearly visible on the photos and therefore the airline had enough information to investigate those responsible.