The fu­ner­al of the on­ly monarch most Britons have known in­volves the biggest se­cu­ri­ty op­er­a­tion Lon­don has ever seen.

May­or Sadiq Khan says Mon­day’s state fu­ner­al for Queen Eliz­a­beth II is an “un­prece­dent­ed” se­cu­ri­ty chal­lenge, with hun­dreds of thou­sands of peo­ple pack­ing cen­tral Lon­don and a fu­ner­al guest list of 500 em­per­ors, kings, queens, pres­i­dents, prime min­is­ters and oth­er lead­ers from around the world.

“It’s been decades since this many world lead­ers were in one place,” said Khan. “This is un­prece­dent­ed ... in re­la­tion to the var­i­ous things that we’re jug­gling.”

“There could be bad peo­ple want­i­ng to cause dam­age to in­di­vid­u­als or to some of our world lead­ers,” Khan told The As­so­ci­at­ed Press. “So we are work­ing in­cred­i­bly hard — the po­lice, the se­cu­ri­ty ser­vices and many, many oth­ers — to make sure this state fu­ner­al is as suc­cess­ful as it can be.”

Met­ro­pol­i­tan Po­lice Deputy As­sis­tant Com­mis­sion­er Stu­art Cundy said the “huge­ly com­plex” polic­ing op­er­a­tion is the biggest in the Lon­don force’s his­to­ry, sur­pass­ing the Lon­don 2012 Olympics.

More than 10,000 po­lice of­fi­cers will be on du­ty Mon­day, with Lon­don bob­bies sup­ple­ment­ed by re­in­force­ments from all of Britain’s 43 po­lice forces. Hun­dreds of vol­un­teer mar­shals and mem­bers of the armed forces will al­so act as stew­ards along the pro­ces­sion­al route.

They are just the most vis­i­ble part of a se­cu­ri­ty op­er­a­tion that is be­ing run from a high-tech con­trol cen­ter near Lam­beth Bridge, not far from Par­lia­ment.

Street drains and garbage bins are be­ing searched and sealed. On Mon­day there will be po­lice spot­ters on rooftops, snif­fer dogs on the streets, ma­rine of­fi­cers on the Riv­er Thames and mount­ed po­lice on horse­back.

Fly­ing drones over cen­tral Lon­don has been tem­porar­i­ly banned, and Heathrow Air­port is ground­ing scores of flights so that air­craft noise does not dis­turb the fu­ner­al ser­vice.

Au­thor­i­ties face the chal­lenge of keep­ing 500 world lead­ers safe, with­out ruf­fling too many diplo­mat­ic feath­ers. Pres­i­dents, prime min­is­ters and roy­al­ty will gath­er off­site be­fore be­ing tak­en by bus to the abbey — though an ex­cep­tion is be­ing made for U.S. Pres­i­dent Joe Biden, who is ex­pect­ed to ar­rive in his ar­mored lim­ou­sine, known as The Beast.

An­oth­er chal­lenge is the sheer size of the crowds ex­pect­ed to gath­er around West­min­ster Abbey and along the route the cof­fin will trav­el af­ter the fu­ner­al, past Buck­ing­ham Palace to Hyde Park. From there it will be tak­en by hearse about 20 miles (32 kilo­me­ters) to Wind­sor, where an­oth­er 2,000 po­lice of­fi­cers will be on du­ty.

The queen is due to be in­terred in St. George’s Chapel at Wind­sor Cas­tle along­side her hus­band Prince Philip, who died last year aged 99.

Po­lice are de­ploy­ing more than 22 miles (36 kilo­me­ters) of bar­ri­ers in cen­tral Lon­don to con­trol the crowds, and tran­sit boss­es are prepar­ing for jam-packed sta­tions, bus­es and sub­way trains as 1 mil­lion peo­ple flood the cer­e­mo­ni­al heart of Lon­don. Sub­ways will run lat­er than nor­mal and train com­pa­nies are adding ex­tra ser­vices to help get peo­ple home.

While many will be mourn­ing the queen, sup­port for the monar­chy is far from uni­ver­sal. Po­lice have al­ready drawn crit­i­cism for ar­rest­ing sev­er­al peo­ple who staged peace­ful protests dur­ing events re­lat­ed to the queen’s death and the ac­ces­sion of King Charles III.

Cundy said it had been made clear to of­fi­cers that “peo­ple have a right to protest.”

“Our re­sponse here in Lon­don will be pro­por­tion­ate, it will be bal­anced, and of­fi­cers will on­ly be tak­ing ac­tion where it is ab­solute­ly nec­es­sary,” he said.

Met­ro­pol­i­tan Po­lice Com­mis­sion­er Mark Row­ley said the goal was to keep the event safe, “and try to do it in as un­ob­tru­sive a way as pos­si­ble, be­cause this is ob­vi­ous­ly a solemn oc­ca­sion.”

Dean of West­min­ster David Hoyle, who will con­duct the fu­ner­al ser­vice in the 900-year-old abbey, said prepa­ra­tions were go­ing smooth­ly — de­spite the oc­ca­sion­al se­cu­ri­ty-re­lat­ed glitch.

“There was a won­der­ful mo­ment when I had flower arrangers wait­ing in the abbey, and no flow­ers, be­cause, quite prop­er­ly, the po­lice didn’t rec­og­nize what the van was and the flow­ers were sent back,” he said.