The area includes the Seven Sisters chalk cliffs near Eastbourne
The South Downs became England's newest national park at midnight.
The area, which covers more than 600 sq miles of countryside in Sussex and Hampshire, is England's tenth national park.
A national park authority has been established which will have increased planning powers and the ability to target resources.
The designation will also mean the area will be eligible for extra funding from central government.
Richard Shaw, interim chief executive for the South Downs National Park Authority, said: "I'm delighted that we have reached this important milestone.
"The South Downs has been nationally recognised for its natural beauty and the space it offers for people to enjoy the countryside. This is why it has been made into a National Park."
The area was awarded national park status in March 2009 and includes the Alice Holt site in east Hampshire as well as Green Ridge in Brighton.
Mr Shaw added: "We have been struck by the love of the South Downs that people have expressed to us from across the region.
"Over the next two to three decades, there will be intense pressures on the downs area - from development to climate change.
"The South Downs now has the highest level of landscape protection afforded to an area in the UK and National Park designation will help to keep the South Downs special."