Members of several anti-nuclear groups who are part of the Stop New Nuclear alliance say they are barring access to Hinkley Point power station in Somerset in protest against EDF Energy's plans to renew the site with two new reactors.
The new reactors at Hinkley would be the first of eight new nuclear power stations to be built in the UK.
Stop New Nuclear spokesman Andreas Speck said: ''This is the start of a new movement. We intend this day to be a celebration of resistance against the Government and EDF Energy's plans to spearhead the construction of eight new nuclear power plants around the UK.
'This is blockade shows that people who understand the true dangers of nuclear power are prepared to use civil disobedience to get their voice heard.
"The Government has hoodwinked the public into believing that we need nuclear power to keep the lights on. But this is totally untrue."
The protesters said they began their blockade at about 7am, with a theatrical troupe who "enacted a nuclear disaster scenario similar to Fukushima", the power plant which was badly damaged during the earthquake which struck Japan on March 11 this year.
Most are local people but demonstrators have also come from Belgium and Germany, a spokesman said.
Hinkley was one of eight sites the Department of Energy and Climate Change (DECC) confirmed in June as being suitable for new nuclear power stations to be built.
The others are Bradwell, Essex; Hartlepool; Heysham, Lancashire; Oldbury, South Gloucestershire; Sellafield, Cumbria; Sizewell, Suffolk; and Wylfa, Anglesey.
A spokesman for EDF Energy said the number of protesters was closer to 100 and they had not chained themselves to any structures at the entrance to the site.