Tens of thousands of free tickets for part of Londonderry's Halloween festival have been claimed within three days of becoming available.

Organisers Derry City and Strabane District Council said more than 25,000 of the 36,750 tickets for the Awakening the Walled City Trail had been booked by Thursday. The tickets became available at noon on Monday.

The trail runs along the city's historic walls and is a central part of the festival, which this year marks its 40th anniversary. The council introduced ticketing for the first time this year due to concerns about crowd control in previous years.

Aisling McCallion, the council's festival and events manager, said the ticketing system is intended to control numbers on the walls to make the experience safer and more enjoyable. She noted the walls are more than 400 years old and were not built for large-scale events.

Tickets are only required for the section on the walls. The rest of the walled city will remain free and non-ticketed, McCallion said.

The Awakening the Walled City Trail runs for the first three days of the four-day festival. A number of tickets will be held back and released on each day of the festival.

Derry mayor Grace Uí Niallais said the trail captures the creativity and cultural heritage that have made the festival internationally renowned.

More than 100,000 people attended the Halloween celebrations in Derry last year. Official civic celebrations began in 1986, and the festival has grown from a local event inspired by a city centre pub's fancy dress night in the 1980s.

Tickets can be booked at the Millennium Forum box office or via its website.