Up to 1,500 people objecting to controversial power and wind energy plans in mid Wales are expected at a protest in Cardiff Bay.
Four campaigners against a network of pylons will end a six-day walk from Welshpool, Powys, and be joined by opponents of wind farms at the Senedd.
Dozens of pylons, some measuring 154ft (47m), and a substation are earmarked to connect with about 10 wind farms.
The Welsh Government said it was determined to cut carbon emissions.
Politicians from the four main parties in mid and west Wales, including Montgomeryshire MP Glyn Davies, who called for the protest, will address the crowd in Cardiff Bay, and three petitions will be handed to the Welsh Government.
Organiser Richard Bonfield said: “We are expecting about 1,500 people from Powys, Shropshire, Ceredigion and Carmarthenshire at the protest. There will be between 20 and 25 buses making the journey to Cardiff.”
In 2005 the Welsh Government unveiled seven areas across mid and south Wales, known as Tan 8, which had been chosen for wind farm development.
In mid Wales, it could mean a line of 50 metre (164ft) pylons carrying 400,000 volt electricity cables from Montgomeryshire to near Shrewsbury.
Tan 8 was part of the UK Government’s energy policy to increase the amount of electricity from renewable sources to 10% by 2010, but Wales has exceeded this and produces about 13%.
“We are calling on the Welsh Government to review the Tan 8 policy and impose a moratorium on wind farm development while the review takes place,” said Mr Bonfield.
Ahead of the demonstration, the former Liberal Democrat Montgomeryshire MP Lord Carlile of Berriew, called the power plans “an unnecessary and an economic error of high magnitude”, and also urged the Welsh Government to review Tan 8.
Lord Carlile, a deputy high court judge and QC, said: “From the time I was MP for Montgomeryshire, I have been opposed to the ruination of the Montgomeryshire landscape by extensive wind farms and their infrastructure.
“Tan 8 fails utterly to take these important issues into account.
“The contribution of onshore wind, and such value as it presents economically, have been exaggerated in order to give the impression that there is a more mixed energy policy than is revealed by true examination of the underlying policy.”
The Welsh Government said it was “determined to see Wales cut its carbon emissions and use more renewable energy”.
A spokesman added: “As well as wind – biomass, marine and micro generation sources, all have their part to play.
“Our planning policy, informed by independent research, identifies seven areas for the development of wind farms to minimise the proliferation of large scale wind farms. We believe a small number of wind farms clustered in strategic areas is better than a large number of smaller wind farms across Wales.”
He said all planning applications for wind farms were subject to a strict planning process which considered “their impact on the environment, and local community”.
The National Grid urged people to take part in the public consultation process which has been extended until 20 June.
Wind turbines have long faced criticism from some local people, but complaints have grown since plans were unveiled for a 19-acre substation in Powys, and dozens of pylons.
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