The Grand National got under way following a 12-minute delay after protesters were removed from the racecourse at Aintree.
The race started at 5.27pm after around 15 demonstrators were reported to have been on the track, clinging to some of the 30 fences.
Horses were taken away from the parade ring while racegoers were informed of the delay.
The Animal Rising group shared on social media that demonstrators had run onto the track, “delaying the race indefinitely”.
“In the UK alone, 49 horses have died or been killed so far this year due to horseracing: this is unacceptable,” the Twitter post read.
“We are a nation of animal lovers – we shouldn’t be harming animals for entertainment, food or any other reason. It’s time to create a kinder future.”
It follows the group announcing plans earlier to prevent the steeplechase from starting, with up to 300 activists planning to scale fences and enter the track.
It said: “We will be slow marching around the perimeter and at some point we may peacefully try to make our way towards the track, again to prevent this race from happening because we know horses are being harmed.”
Ahead of the Grand National race, three people were arrested on suspicion of conspiracy to cause a public nuisance, including 25-year-old spokesperson Claudia Penna Rojas.
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A man and a 33-year-old woman were detained in Greater Manchester on suspicion of the same offence.
The presence of protesters did not deter racegoers from attending the event as the first race got under way at 1.45pm.
Emma Simpson, from Eastham in Wirral, who was at Aintree to celebrate her 40th birthday, said: “There’s ways of having a protest and ways of doing that and having an impact on people without disrupting everybody else.”