British authorities are "working tirelessly" to confront the terrorist threat, the country's most senior counter-terrorism officer has said.
Assistant Commissioner Mark Rowley said Scotland Yard is "constantly testing" London's preparedness to respond to an attack.
He was speaking ahead of a meeting of security experts, police and other figures to discuss arrangements in the capital at London's City Hall.
The threat to the UK from international terrorism currently stands at severe - indicating that an attack is seen as "highly likely".
Earlier this year it was revealed that authorities have disrupted seven plots to attack the UK in the previous 18 months.
Mr Rowley said: "The Met is constantly testing and exercising London's preparedness and this will continue for as long as the threat level remains at severe.
"We are working with London's local authorities and other key partners to test our preparedness in the event of a successful attack getting through.
"This is part of the UK's ongoing contingency planning which the public would expect us to fulfil at this time of heightened threat.
"The Met and national counter-terrorism policing network are working tirelessly with partners to confront the terrorist threat.
"This includes arresting those who are planning, preparing or funding terrorism; working with communities to prevent people being drawn into violent extremism, and carrying out protective security operations to make London and the UK a hostile environment for terrorists to operate in.
"Today's event is a key part of our work to protect Londoners and keep them safe."
Lord Toby Harris of Haringey has been appointed by London Mayor Sadiq Khan to carry out a review into London agencies' ability to cope with multiple simultaneous incidents.
Lord Harris said: "Our agencies and emergency services work hard every single day to keep Londoners safe. Meetings like today's are vital as part of our ongoing contingency plans and to see whether existing arrangements can be made even more robust."
Sophie Linden, Deputy Mayor for Policing And Crime, said: "The Mayor's top priority is keeping Londoners safe, and partnership working is vital to ensure a fast, effective and streamlined multi-agency response in the event of a terrorist attack."