Thousands of people are in Derry for the 2016 “Out of this World” Halloween festival.
A number of road closures are in place throughout the city centre, but who needs transport infrastructure when you’re a ghost or a ghoul?
Even a slight drizzle earlier in the evening did nothing to dampen spirits.
The parade in the renamed City of Bones began at 19:00 GMT, followed by fireworks from 20:00 GMT.
Londonderry now lays claim to being the “best Halloween destination in the world” having been voted number one in a USA Today readers’ poll last year.
The festival, organised by Derry City and Strabane District Council, is now in its 30th year.
One of the organisers told the BBC an estimated 30,000 people had gathered in the city for the culmination of a three-day festival, which had already attracted an estimated 40,000 people during the weekend.
Ch Insp Gordon McCalmont, who is in charge of the policing operation, says about 200 officers are involved.
He said the operation was “not without its challenges” but it was also “thoroughly enjoyable”.
At the spooky scene
By BBC NI’s Eimear Flanagan
The streets of Derry are thronged with people who have come out to join the city’s biggest-ever Halloween party.
Guildhall Square is alive with the sights, sounds and smells of celebration as musicians and dancers entertain the crowds and the scent of street food stalls waft through the air.
Many have come dressed for the occasion and some have put a real effort into their costumes.
They include Derry Halloween festival regular Elaine Maguire, who dresses up as a different Disney princess every year. Batman, the cast of Ghostbusters and Star Wars characters have also showed up for party.
Last year, about 50,000 people attended the festival and the organisers are confident that will be exceeded tonight.
An invitation was extended to extra-terrestrial guests, and a World War Two siren wailed across the city to “prepare earth residents for their arrival”.
After the parade ended at Queen’s Quay, the fireworks provided a spectacular finale for the entire Derry and Strabane area, with the council spending £30,000 on the display.
There will be music in Guildhall Square until 22:00 GMT.
“The world knows no-one throws a Halloween party like us,” said Mayor of Derry City and Strabane District Council Hilary McClintock.
“For 30 years now, people from all over the world have visited our city and celebrated with us in our unique Halloween celebrations, making memories and great friends during their visit.”
Jacqueline Whoriskey, from Derry City Council, told the BBC it cost about £220,000 to put on the Halloween celebrations across the district.
It is not just all about 31 October – the city has been partying hard for the last three days.
Derry’s Halloween festival officially began on Friday, with a packed weekend programme of “freaky family fun”.
There was live music, street performances, ghost tours, spooky story-telling, fancy dress dances, a food and craft market and a 5km “zombie run”.
The city’s historic 17th Century walls were a dramatic backdrop to the weekend carnival, but proved no defence from the alien invaders.
The Awakening the Walls exhibition was billed as a “supernatural animation trail” featuring beautiful illuminations by local and visiting artists.
On Monday night, Derry is making full use of its position on the banks of the River Foyle to give its visitors a whale of time.
It’s all a far cry from the festival’s humble beginnings 30 years ago.
Earlier this month, council worker Seamus Carlin shared his memories of Derry’s first Halloween carnival in 1986.
“It started as a small event in the Guildhall Square with some fireworks,” the building maintenance foreman recalled.
“It took us six hours to build the ‘shamrock stage’ on the first year of the event.”
He added: “I enjoyed the fact that everything was authentic back then, from the handmade costumes to the specially crafted designs.
“I’ve seen everything from witches to five Embassy Regal [cigarettes] entering the square over the years.”
The BBC News NI Live team is in the city and on air until the later time of 21:00 GMT, to capture all the fun of the festival.
You can check out the full programme of events on the council’s website.