* London police call in national assistance to guard airport
* Interior minister tells police to adopt "firm stance"
* Number of arrests this week rises above 500 (Updates number of arrests, adds protester quote and context)
By Emily G Roe and David Milliken
LONDON, April 18 (Reuters) - The Extinction Rebellion group of climate change campaigners said it planned to hold a protest at London's Heathrow Airport on Friday, opening a new front in its demonstrations that have caused transport snarl-ups in the British capital.
Extinction Rebellion has blocked several locations in central London in recent days after it staged a semi-nude protest in parliament earlier this month.
More than 500 people have been arrested this week and 10 charged so far, police said.
London's police force said it had cancelled some officers' leave and was calling in assistance from other forces to deal with protesters who were causing "unacceptable" disruption.
Extinction Rebellion sent a message to media titled "Statement on the Extinction Rebellion Heathrow Action Tomorrow" but the body of the message did not give further protest details.
The group has called for non-violent civil disobedience to push the British government to reduce net greenhouse gas emissions to zero by 2025 and to stop what it says is a global climate crisis.
"If we don't do something now it's going to have a catastrophic effect," said 23-year-old media student Fflur Harman, who had travelled from central England and spent the night at one central London protest site.
The pace of reduction in emissions called for by Extinction Rebellion is far faster than that urged by the United Nations' Intergovernmental Panel on Climate Change, which last year recommended they be cut to zero on a global basis by 2050.
Britain has lowered net emissions by 42 percent since 1990, and currently aims to cut emissions by 80 percent by 2050. Government advisors will suggest new targets next month.
Heathrow said it was working with authorities to address any threat of protests which could disrupt the airport on one of the busiest travel days of the year as the Easter getaway begins.
"While we respect the right to peaceful protest and agree with the need to act on climate change, we don't agree that passengers should have their well-earned Easter Break holiday plans with family and friends disrupted," the airport said.
London's Metropolitan Police said it had "strong plans in place that would enable it to deploy a significant number of officers to Heathrow and take firm action" if needed.
Interior minister Sajid Javid said he wanted police to "take a firm stance and use the full force of the law".
However, police said they were limited in the action they could take as the protests were disruptive, rather than violent.
"The question really is can we arrest our way out of this issue, given there are several thousand people in London who are willing to be arrested," Metropolitan Police Assistant Commissioner Nick Ephgrave told broadcaster Sky News. (Additonal reporting by Michael Holden Writing by William Schomberg and David Milliken Editing by Andrew Heavens)
Our Standards: The Thomson Reuters Trust Principles.