Police officers are bracing for a weekend of carnage at the Notting Hill Carnival as they install knife arches and live facial recognition cameras to catch criminals.Arrests have already been made ahead of Europe's largest street carnival as officers fear another weekend of violence after last year's record-breaking arrests total.Some 7,000 Metropolitan Police officers and staff will be on duty to keep an eye on two million revellers who are set to descend on the streets of west London.More than 3,000 stewards will be on duty - double the usual number - after crowd control responsibilities were transferred from Scotland Yard to the event organisers following an extra £4.66million grant announced by Sir Sadiq Khan in March.But Susan Hall, leader of the Conservative group at the London Assembly, said she remained concerned about the security risks and called on the Mayor to 'get a grip on this once and for all before more people are injured or die needlessly'.Metropolitan Police Federation chair Paula Dodds warned officers were 'already stretched beyond belief', adding that their 'welfare must remain a top priority'.The Met said last year's policing operation cost about £15million – and while the force hopes this year's total cost will be lower, this 'still represents a very significant financial outgoing in the context of unprecedented budget challenges for policing'. The force said operations involving live facial recognition and numberplate checks were being carried out to locate wanted offenders before and during the carnival.Police are also making arrests of gang members and Violence Against Women and Girls (Vawg) suspects. There is also a ban in bail conditions and prevention orders for high-risk people attending the event, especially domestic abusers and sex offenders.Weapon sweeps of known gang areas are being made, 'knife arches' will be in use and there will be increased uniformed police patrols in parks and open spaces.Detectives are also carrying out targeted checks on hotels and short-stay accommodation to catch people involved in child exploitation and county lines.Organisers said there will be increased safety and crowd control measures for the carnival, which is marking its 60th anniversary – including a bigger control room, more CCTV cameras to monitor revellers and signs to alert them to possible hold-ups. Police officers observe revellers at the Notting Hill Carnival in west London in August last year