Subway drivers in London are staging a 24-hour walkout, making it difficult for thousands of shoppers out to take advantage of the traditional post-Christmas Day sales.

The London Underground train drivers' union is seeking higher wages for the drivers. They are angry that they have not been given extra pay and a day off for working on December 26, a public holiday in Britain known as Boxing Day.

Authorities warned shoppers that they would face severe disruptions to service, with many subway lines shut down or operating at vastly reduced service. People were forced to use buses or taxis to reach shops that are desperate for business after disappointing sales in recent months.

Despite the strike, large crowds of shoppers flooded department stores in London and other British cities as soon as doors opened early Monday.

The drivers' union has promised three more strikes in January and February if the dispute is not resolved.