The annual Apprentice Boys of Derry parade will take place today, with significant traffic disruptions possible.
Police say this is the largest loyal order parade in Northern Ireland with 15,000 people and 140 bands expected this year.
Diversions will be put in place from around 10.30am in various points around the city centre, and traffic will not be allowed onto the parade route after 12 noon.
There have already been a number of feeder parades, with the main parade getting underway at 12.30.
A large firework fired from a boat on the Foyle to symbolise the breaking of the boom will signal the start of the main parade.
The various clubs and their supporters will gather in the Duke Street/Spencer Road area from around 11.00am. The parade will be on the move from 12.30pm until 5.15pm with rolling diversions and traffic restrictions in place.
After the main parade, smaller parades are expected in several towns this evening, including Plumbridge, Castlederg, Newtownstewart, Ardstraw and Donemana as local Clubs return home.