Poilce said the “violent disorder and assault” in Small Dean, Wendover last Friday night left nine members of site security staff injured.
HS2 chiefs said the attack was the part of a recent campaign of intimidation and abuse by activists which has forced them to step up security on the job.
Main contractor on the job is the EKFB joint venture of Eiffage, Kier, Ferrovial and BAM Nutall.
An HS2 spokesperson said: “Over the last few weeks, our staff working near Wendover have been subject to a campaign of abuse, harassment and violent assault.
“On Friday night, this campaign escalated, with a gang of 30 masked activists attacking staff, punching and stamping on them in the dark in the middle of the A413, leaving eight people injured and one taken to hospital.
“This is totally unacceptable and we are cooperating with the police to bring the perpetrators to justice.
“In the meantime, extra security is being put in place including more lighting, fencing and dog patrols.”
Much of the incident occurred on the A413 with passing traffic narrowly missing people on the road.
A spokesman for Thames Valley Police said: “This was an unacceptable outbreak of violent disorder where a number of people were attacked in what appears to have been a pre-meditated and orchestrated incident.
“This incident took place in close proximity to the A413, and although thankfully no one was seriously hurt, this was a serious incident.
“We are appealing to anybody who was driving in the area at around the time and who has dash-cam in their vehicles to please check this and contact the police if it has captured anything that could assist this investigation.
“You can call 101, quoting incident reference 2343 of 26 March or make a report online.
“You can also report 100% anonymously via the independent charity Crimestoppers on 0800 555 111.”
Local residents said tension between HS2 security staff – dubbed locally as “carrots” because of their orange vests – and activists at an adjacent protest camp has been simmering for months.