Mr Parkes was leaving Superstar Boudoir when he was attacked
A gang of up to 20 youths attacked an off-duty trainee police officer outside a gay bar in Liverpool city centre.
James Parkes, 22, who is training with the Merseyside force, was out with friends when he was set upon by the gang on Stanley Street on Sunday night.
Four boys from the Kirkdale area, one aged 17, two aged 15 and one 14, have been arrested on suspicion of assault.
Mr Parkes is in hospital with multiple skull fractures and fractures of his eye-socket and cheek bone.
He was leaving Superstar Boudoir, one the city's well-known gay bars, when he was attacked.
His condition was described as "very critical".
A spokesman for Merseyside Police said: "The officer, who is openly gay, was with his partner, another man and a woman when the incident took place.
"From the language used by the offenders towards the officer we perceive it was a homophobic attack."
One of Mr Parkes' companions was punched in the face.
Mr Parkes was a police community support officer earlier this year.
He began his initial 31 weeks training in May and is due to pass out in early 2010.
High visibility patrols have been stepped up in the area and CCTV footage of the scene is being reviewed.
Det Ch Insp Tim Keelan, leading the investigation, said: "Help us to catch those responsible for this despicable crime.
"We are treating the assault as a homophobic hate crime and this incident shows there are still some people who have not learnt that crimes of this type are completely unacceptable."
Stanley Street has been cordoned off while officers carry out forensic examinations.