Hundreds of protestors have gathered on Parliament Square to demonstrate against a third runway at Heathrow.
David Cameron is due to make a decision about expanding either Gatwick or Heathrow in a few weeks time.
All 5 London Mayoral candidates are expected to speak at the event.
A rally against plans to build a third runway at Heathrow Airport is due to take place later this morning in Parliament Square.
All the candidates to succeed Boris Johnson as Mayor of London from the main political parties are expected to attend, including the Labour MP for Tooting Sadiq Khan and his Tory rival, Richmond MP Zac Goldsmith.
Earlier this year, the Airports Commission recommended expansion at Heathrow Airport as the best way to increase airport capacity for London and the south east.
However, London Gatwick has continued to make the case for why it should be allowed to expand.
A number of east London MPs have written to the Prime Minister claiming London will 'fall behind' unless development goes ahead.Read the full story ›
Tens of millions of pounds is being spent giving the road tunnels into Heathrow a makeover to improve safety and allow a new generation of heavier planes to travel on the runway which goes over them.
The tunnels off the M4 were built in the 1950s and are used by 55,000 vehicles a day.
Now the two 630m-long access routes are being refurbished in overnight works to allow the airport to operate as normal during the day.
New fire systems are being installed and they're being made stronger to cope with a new generation of larger planes like the A380.
Residents around Heathrow are furious over today's announcement that the airport is the preferred option for a new runway. Interviewees: Helen Hansen-Hjul and Murray Barter.
Boris Johnson has told ITV News that the Airport Commission's proposals for a third runway at Heathrow will lead to a massive increase in noise and air pollution over London and suggested little has changed since the West London airport's first plans for redevelopment three years ago.
The Mayor of London seemed wary that the planned expansion would lead to the erosion of a night flight ban and eventually a fourth runway, which Mr Johnson said would be a "real disaster".
Conservative MP Zac Goldsmith, who is seeking nomination as the party's candidate for London mayor and has been a vocal opponent of a third runway, said:
It's not going to happen. I don't think anyone believes Heathrow expansion is possible or deliverable politically. There are conditions in this report on air quality and noise pollution that are irreconcilable with Heathrow expansion.
Mr Goldsmith said he would honour his pledge to stand down as an MP and force a by-election in his Richmond Park constituency if a Conservative government gives the green light to the third runway option.