A "catastrophic price" will be paid if a third runway is built at Heathrow airport, the Tory London mayoral candidate has said.
Zac Goldsmith told a rally in London's Parliament Square that expanding the UK's largest airport would make the capital's air pollution problems "unsolvable".
The MP for Richard Park said: "We know a million homes, a million families, would be subjected to noise way beyond what is acceptable, according to the World Health Organisation.
"We know that our air pollution problems in London would be unsolvable if we expand Heathrow. And we know it requires the demolition of more than 1,000 homes.
"It is a catastrophic price to pay."
Mr Goldsmith described the Airports Commission's report which recommended the expansion of Heathrow as "discredited", "bogus" and "hopeless". Hundreds of opponents to a third Heathrow runway gathered outside the Palace of Westminster as London mayoral candidates and local MPs addressed the rally.
They chanted "no ifs, no buts, no third runway" in reference to Prime Minister David Cameron's comments in 2009, when he said: "The third runway at Heathrow is not going ahead, no ifs, no buts."
Building a third runway at Heathrow was recommended in July by the Airports Commission, which rejected expansion of Gatwick. Heathrow was chosen by the commission as the best way to increase airport capacity for London and the south east because it was predicted it would add £147 billion in economic growth and 70,000 jobs by 2050.
Mr Cameron is due to announce a decision on expansion by the end of the year.
Organisers of the rally said London mayor Boris Johnson was due to speak at the event but he could no longer attend.
A spokesman for Mr Johnson said he was not attending the rally because he had a personal commitment out of the country.
"He would be there otherwise," the spokesman said.
"Boris's support remains undiminished. He wishes them well."
Mr Johnson, who steps down as London mayor next year, had previously proposed a new airport should be built in the Thames estuary but the plan, dubbed "Boris Island", was dismissed by the commission.