Opposition to wind power in rural Wales is said to be "total", with communities threatening peaceful direct action and at least 20 groups fighting plans for 870 of the largest turbines in Powys alone.

"It's only in the last few months that people have begun to understand the scale of what is happening. Even the business community is shocked. We can call on at least 5,000 people now for meetings," said Richard Bonfield, former chair of mid-Wales Confederation of British Industry (CBI), who has helped organise protest rallies at the Welsh assembly in Cardiff and elsewhere.

"This is one of Wales's biggest ever infrastructural projects and no one is now in favour except a few farmers. In the past we had a few small farms and turbines: we could accept them. But now we're talking 120-metre-tall turbines and giant pylons. It is an environmental disaster."

According to protesters, it is the applications by the National Grid company to build a 7.5-hectare (19-acre) substation at Abermule to link several major new windfarms in mid-Wales to the grid, and plans to erect possibly 25 miles of 46-metre, 400,000v pylons across lowland areas of Powys, Shropshire and Snowdonia, that have done the most to inflame passions.

"The pylons have made people sit up. A few years ago it was really only the people who could see the farms who were in opposition. It has now broadened out as people see the true scale of the developments," said Caroline Evans, of the Brechfa Forest Energy Action Group near Carmarthen.

She expects her village to be surrounded by four windfarms and dozens of pylons: "Everyone is now affected. People are very fed up."

Anger in mid-Wales at pylons and farms has spread north to Snowdonia and Anglesey, where hundreds of people have packed recent meetings called by protest groups to debate applications for more than 50 turbines.

Glyn Davies, the Conservative MP for Montgomeryshire, was heavily criticised last year for backing a Welsh anti-pylons direct action group. He said the entire region was now "up in arms" against the pylon and turbine plans. "They will industrialise the uplands with wind turbines and desecrate our valleys with hideous cables and pylons. The scale is almost impossible to comprehend," he said.

National Grid says it will need to erect a 15-mile row of Britain's biggest pylons to connect a huge offshore windfarm off Anglesey to the grid, as well as a second nuclear reactor at Wylfa. Another line of pylons will be needed to Trawsfynydd.

"No one knows the full implications of these developments or what they will do to tourism. The opposition is total and very impressive," said Welshpool campaigner Richard Jones. "I was broadly in favour of wind power until I saw the scale of the pylon developments."

According to National Grid, 215 miles of the largest 400,000v pylons will be needed by 2020 in England and Wales, as well as new sub-sea cables down the east and west coasts of England and others linking Scottish islands. The new 400,000v pylon lines will affect Kent, Lincolnshire, East Anglia and Somerset as well as Wales. Tens of thousands of people opposed giant pylon plans now being erected through Scotland.

David Mercer, National Grid's head of major infrastructure, said hundreds of miles of pylons erected in the 1960s would have to be replaced, possibly with bigger ones, to accommodate new windfarms as well as a generation of new nuclear power. "We only build where we have to. We work very closely with communities, and have had umpteen forums. But it's hard to please everyone," said Mercer.

"Ultimately, society must decide what is appropriate to spend. It's not for us to decide whether the lines go above or below ground. In sensitive areas, it's likely to be a mix. If all 345km [215 miles] of new lines were to go on overhead cables, it is likely to cost £700m, but it could be £6bn if it were all to be buried. That has to be a decision for government. We are not trying to make a profit from this."

New figures released by the Department of Energy and Climate Change (Decc) expect the cost of adapting the grid to the new generation of nuclear and wind power to be £14bn between 2013 and 2021.