LONDON (Reuters) - Police in Windsor, where Prince Harry marries American actress Meghan Markle next month, said on Thursday they had begun one of the biggest security operations in their history to ensure the event passes off safely.

Harry, 33, and Markle, 36, will tie the knot on May 19 at St George’s Chapel at Windsor Castle, the home of the prince’s grandmother which dominates the town about 20 miles (30 km) west of London.

More than 100,000 visitors are expected to descend on Windsor to celebrate the wedding and police have said that barriers to stop vehicle attacks, armed patrols, and airport-style security would be deployed to prevent any incident.

Specialist explosive detection dogs from Thames Valley Police searched the town on Thursday, examining mail boxes, phone booths and drains for signs of potential explosive devices.

“This is a large operation for any police force and one of the biggest I have worked on,” said police search coordinator Andy Turner. “We of course want it to all go smoothly.”

Britain is on its second-highest threat level - severe - meaning an attack is considered highly likely. Last year there were five incidents classified as terrorism, in which 36 people were killed.

Police said shops and buildings would be searched in the days leading up to the wedding. British Transport Police also said they would have both covert and highly visible officers across London and southeast England on the day, with firearms units at key stations.

“We’ve worked closely with our partners at other police forces, local authorities and train companies to ensure that we can respond quickly and decisively to any incident – whatever it may be,” said Chief Superintendent John Conaghan.

Bob Broadhurst, the officer who was in charge of policing the 2011 wedding of Harry’s elder brother William to wife Kate, has said there several issues police would have to be prepared for.

“It is impossible to search tens of thousands of people who want to come and line the streets, any one of which could be a potential terrorist; someone who’s besotted with the royal family - and there are a number of those; or just an attention-seeker,” he told Reuters earlier this month.

He said officers had dealt with 18 protests on the day of the 2011 wedding, which passed off without incident, but there were complaints from campaigners that police had been heavy-handed.

The anti-monarchist Republic group said it had written to the local police chief asking for assurances peaceful protests would not be stopped this time.

“The monarchy is a contested institution and, while this is a private wedding, all major royal events such as this are used as PR for the monarchy,” wrote Graham Smith, Republic’s chief executive.