The start of the Grand National Festival was delayed after a number of animal rights protesters made their way on to the track at Aintree Racecourse.
Climate and animal rights group Animal Rising had threatened to disrupt the four-and-a-quarter-mile race and were protesting from early this morning outside the track.
As the National runners were in the parade ring, a number of protesters breached security fences around the Liverpool venue and ran on to the course.
The horses were sent back to the pre-parade ring, with the race facing an indefinite delay.
Merseyside Police said three people had been arrested on suspicion of conspiracy to cause a public nuisance, with a 25-year-old woman from London and a 55-year-old man from Greater Manchester arrested outside the racecourse today.
A 33-year-old woman from the London area was also arrested in the Greater Manchester area earlier in the day.
A spokesperson for Merseyside Police said: "Merseyside Police has been working with The Jockey Club and other partners to keep people safe during the Grand National Festival.
"We are aware of some people planning to protest at the event. This has been factored into our plans. We respect the right to peaceful protest and expression of views, but criminal behaviour and disorder will not be tolerated and will be dealt with robustly."
About 30 animal rights protesters gathered outside Aintree Racecourse, with the annual Grand National race set to start at 5.15pm.
Dora Hargitai, 37, a volunteer with Animal Rising, said: "I do believe we can have non-violence on both sides.
"The race has to stop. Today and forever."
Claudia Penna Rojas, from Animal Rising, said: "The reason we're here today is first and foremost for these horses.
"We're animal lovers, as I'm sure are most people here today, and we want to protect them and keep them safe and unfortunately with events like the Grand National and when we exploit animals in general for entertainment or for food we are putting them directly in harm's way."
She said the group would be demonstrating outside the course.
She added: "We will be slow marching around the perimeter and at some point we may peacefully try to make out way towards the track, again to prevent this race from happening because we know horses are being harmed."
She said if activists did get onto the track it would not be while horses were running because they did not want to put them in danger.
