A Mail on Sunday investigation said the Animal Rebellion group was aiming to "storm security fences" before the main race in Liverpool on 15 April.
They claimed protestors plan to form a barricade across Aintree Racecourse.
Animal Rebellion said they were launching their "biggest campaign" this April.
In a statement, Rose Patterson, from the group, said the Mail investigation was "not going to stop" members from "beginning the crucial conversation about our broken relationship with other animals".
"The Grand National is symbolic of this broken relationship," she added.
Two horses died in last year's Grand National race, which is the climax of the annual three-day racing festival at Aintree.
There have been four fatalities from 356 runners in the nine Grand Nationals raced since safety changes were introduced in 2012.
Animal Rebellion, which calls for an end to horse racing, said: "This April we are launching the biggest campaign to protect animals and the climate this country has ever seen."
They said more than 500 people had "signed up for training to take peaceful action this summer".
In a statement, Merseyside Police said: "We respect the right to peaceful protest and expression of views, but public order or criminal offences will not be tolerated and will be dealt with robustly."
They added they had been working with The Jockey Club "for a number of months… to ensure that any necessary plans and processes are in place to deal with any incidents that may arise".
Race organisers at The Jockey Club have declined to comment.
Animal Rebellion has risen to prominence in recent months after protests at high-end restaurants, dairy suppliers and laboratories.
They say they want a "plant-based future" and have called for an end to horse racing, animal farming and fishing.
The Mail investigation said more than 100 people have signed up to the attempt to disrupt the Grand National and some activists had conducted two reconnaissance missions around the racecourse.
They claimed the group planned to use ladders to get over the security barrier and to cut through the fence before the main race at the end of the three-day festival.
A key event in the national sporting calendar, the Grand National attracted a peak of 7.5 million viewers in the UK and more than 12 million bets last year.
There have been small protests outside the course regularly and occasionally individuals have got on the track itself.
In 1993, the Grand National was delayed after an animal rights protest, with the race eventually voided after a number of false starts.
The race was postponed in 1997 when 60,000 people had to evacuate the course following an IRA bomb threat minutes before the start.
It was eventually held two days later.
This year's event coincides with the 34th anniversary of the 1989 Hillsborough disaster, which led to the deaths of 97 football fans after a crush at a game between Liverpool and Nottingham Forest at Sheffield Wednesday's ground.
Race organisers said they plan to hold a commemoration for the victims at the course on the day of the Grand National.