A protester was detained by police on the racecourse during the Epsom Derby, but failed to stop the race.
Police said officers acted “quickly and decisively” after a man jumped the fence during the day’s main event.
Animal Rising, which had publicly threatened to disrupt the event, claimed the race was allowed to start despite the protester being on the track, which it said was “sickening”.
More than 30 protesters were arrested on Saturday.
Last week, race organisers the Jockey Club won a High Court injunction to prevent animal rights protesters disrupting the event.
A huge security operation has been under way throughout Saturday, including pre-emptive arrests in the early hours of the morning.
Despite security and police officers being stationed around the racecourse, one man was able to break through the cordon while the race was in progress, and another woman was detained while trying to climb over the fence.
Activist group Animal Rising shared a video on Twitter of the protester who entered the track being bundled to the floor by security and quickly dragged away.
Racegoers could be heard jeering at the protesters and shouting abuse.
Jack Cummins, 23, from Cambridge, witnessed the protest. He told the PA news agency: “The guy started running down the course from the hill side – to be fair to security and police, they grabbed him pretty quickly and dragged him off.
“Pretty stupid of him to do it, protesters don’t understand how well-treated these horses are – they’re putting the horses in more danger than they’ll have ever been in by jumping into the track.”
Nevin Truesdale, head of the Epsom Jockey Club, said the protester’s actions were “reckless and illegal” and “threatened the safety of our equine and human athletes”.
It’s unclear from the footage if the race had already began when the man entered the track, but Animal Rising claimed it was allowed to start once the protester was already on the course.
British Horseracing Authority (BHA_ chief executive Julie Harrington said he ran onto the track after the race had started, calling it “reckless and dangerous behaviour”.
A spokesperson said: “Despite The Jockey Club’s claim that the horses safety is paramount, they decided to start the race despite knowing that a protester was on the tracks.
“Another sickening display of profit (over) care for animals.”
Surrey Police said it had detained 12 suspected protesters in the grounds of Epsom racecourse, while 19 were arrested before the event began.
A police spokesperson said 11 people were arrested at addresses in nearby Mitcham and Byfleet after detectives received “intelligence”, and a further eight were arrested after their vehicle was stopped less than three miles from Epsom Downs Racecourse.
All were arrested on suspicion of conspiracy to commit public nuisance and remain in police custody.
In a statement following those initial arrests, Animal Rising accused police of using “heavy-handedness and intimidation tactics” and reiterated their “commitment to protect horses and disrupt the Derby”.
Organisers spent an extra £150,000 on security measures, and there was noticeably more security along the track, with printed-out copies of the High Court injunction attached to the perimeter of the racecourse with plastic coils.
Men who arrived wearing bowler hats were asked to take them off so security could see if there was anything hidden inside them, and a new six-foot fence was installed around the area where members of the public enter the compound.
Tight security included police spotters with binoculars at the top of the Queen Elizabeth stand and other police officers with rifles walking around enclosures and the grounds.
Ryan Moore won his third Derby by guiding Auguste Rodin to victory at Epsom as Frankie Dettori finished 10th on Arrest in his last ride in the race.
The race was moved from its traditional 16:30 time slot to avoid clashing with the FA Cup final at 15:00.
In April, animal rights activists delayed the start of the Grand National by getting on to the track at Aintree.
Protesters also tried to disrupt the Scottish Grand National the following week.
Animal Rising spokesperson Nathan McGovern has previously said: “We are looking to continue the conversation that we started at the Grand National about our broken relationship with animals and nature.
“On the ground we are looking to cause the cancellation or severe delay of the event so that everyone in the country has this discussion.”
