Opponents of a planned M4 relief road, which it is claimed would wreak havoc with a wildlife haven, have staged a campaign event over their concerns.
Gwent Wildlife Trust claims the road would damage the Gwent Levels Sites of Special Scientific Interest (SSSIs).
It had invited residents to a "Marshes or Motorway" event, to explain its fears over the plans.
The Welsh Assembly Government said the proposed route had been realigned in 2004 to minimise impact on the levels.
In September, Transport Minister Ieuan Wyn Jones said the road would run from Magor to Castleton, and could open in 2013.
His comments followed two M4 crashes which led to traffic chaos, including it being closed for eight hours following a head-on collision in which five people died.
Gwent Wildlife Trust (GWT) has urged the public to support its campaign against the proposed road.
It claims the plan contradicts the assembly government's pledge of putting sustainable development at the heart of its agenda.
'Rare flora and fauna'
The "Marshes or Motorway" event, at its Magor Marsh reserve on Tuesday, included tours of the SSSI, it said, and was a chance to question key ecologists and campaigners.
Julian Branscombe, GWT chief executive, said the Gwent Levels was a prime example of wetlands where rare aquatic flora and fauna had survived.
Species there included water voles - "our fastest-declining mammals", rare plants like frogbit, insects like the silver water beetle, said Mr Branscombe.
Its drainage ditches, known as reens, are inhabited by more than 150 "nationally-significant" invertebrates, he said, some of which date back to Roman times.
He added: "Five miles of the wetland would be directly affected by this monstrous road proposal - an ill-conceived and unsustainable relief road for the existing M4 around Newport."
Mr Branscombe said a further worry was that pressure for new developments along the road could result in road kills of rare species like otter and barn owl.
Speaking in September, Ieuan Wyn Jones said a business case for the plans would be looked at and, if it was a "robust business case", there was the possibility of a business inquiry in 2009 and work starting in 2010.
A Welsh Assembly Government spokesperson said the project planning included a series of environmental impact surveys required under environmental legislation.
She added: "If the minister approves the business case for the project, it is expected that a public local inquiry will be held, which will provide an opportunity for an independent review of the project.
"The recommendations from this inquiry will enable the minister to make an informed decision."
Plans for a relief road have existed for 15 years. Other groups to have voiced their concerns over the current proposals include Friends of the Earth Cymru.