Crisis at Christmas welcomed 3,200 guests in 2011.
The charity will provide 21,000 meals at nine centres across the capital between now and next Sunday.
The vast majority of the food is donated and this year includes 17,000 eggs and 4,000 loaves of bread.
For Christmas lunch, 110 turkeys each weighing 10lbs will be served up.
115 cars, minibuses and vans will help distribute food and supplies as well as bring homeless people to the centres.
It's the largest volunteer event in the UK involving 8,700 people from all walks of life, including 250 healthcare professionals.
Among the other services and activities on offer are clothing repair, hairdressing, footcare, housing and legal advice, as well as live music and entertainers.