Hundreds of ramblers took advantage of the "right to roam" as large areas of northern England were opened up.
The launch ceremony was at the spectacular Ribblehead viaduct
Walkers got "open access" to more than 500,000 hectares after a ceremony at Ribblehead in the Yorkshire Dales.
The weekend also marks the beginning of the right of access in Wales as part of the roll-out of access areas.
But landowners have warned the "right to roam" law could falsely lead people to believe they could "drive in to the countryside and then just set out".
Douglas Chalmers from the Country, Land and Business Association said: "What does worry us is that people think this right to roam phrase has been used in the media, that they think they can just get in their cars, drive in to the countryside and then just set out".
The opening of land has also prompted fears among some landowners that farmland will be damaged and privacy infringed.
The "right to roam," granted by the Countryside Act, allows walkers, runners and climbers to roam the access areas. Vehicles are not allowed.
It also excludes access to cultivated farmland and gardens, and roamers must obey local restrictions.
Rural Affairs Minister Jim Knight was joined by Doug Scott, the first Briton to climb Mount Everest, at the Ribblehead viaduct on Saturday.
They then exercised their new rights by heading off across the moors with hundreds of other walkers.
Ramblers also met at the Life Centre, near Ogmore Vale, Glamorgan, to mark the start of Open Access rights in Wales.
The upper North West and North East regions are the fourth and fifth to open in England in a staged roll out of the Countryside and Rights of Way Act, passed by Parliament in 2000.
The area of land to be opened up runs from the Scottish border down to York in the east and Kendal in the west, and from the west to the east coast.
Mr Knight said: "The Yorkshire Dales offers some of the best walking in the country and spectacular Ribblehead valley is a perfect location to mark the opening of the latest and by far the biggest area of access land.
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"The Upper North West and North East comprises of no fewer than four national parks, so this is a great opportunity for everyone who loves our countryside to put their boots on and get out and enjoy it.
"But it is important to repeat the message that people must act responsibly when exercising their new right, especially during the bird-nesting season."
Jacquetta Fewster, director of campaigns for the Ramblers Association, told BBC News 24 it was a "fantastic" day for walkers.
"For the first time ever, we all have the legal right to explore about 4,000 square miles throughout England and Wales, of some of the most stunning countryside."
"It's an absolutely fantastic day, wonderful."
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Her colleague Kate Canto said: "Everyone knows that walking is extremely good for you, and we also think there'll be benefits for the rural economy.
"Hopefully people will go out to these new areas and spend money in pubs, shops, b&bs and businesses and rural economy will benefit".
Tayo Adebowale, of the Countryside Agency, said the area being opened had some of the "most beautiful and dramatic landscapes in the north of England".
"Of course, with the new rights come new responsibilities, so it is important to follow the Countryside Code, and any local signs which may limit where one can walk or take a dog."