"I want to pay tribute to the courage of all the 100 British troops who have given their lives in Afghanistan in the service of their country," Brown said in a statement.
"The risks they bear and the sacrifices they make should be in our thoughts, not just today but every day. They have paid the ultimate price, but they have achieved something of lasting value - helping turn a lawless region sheltering terrorists into an emerging democracy."
Most of the UK troops have died in the past two years in Helmand Province, a volatile southern area where Britain is the main contributor to NATO forces fighting Taliban guerrillas in support of a pro-Western government.
Defence Secretary Des Browne called the conflict the "noble cause of the 21st century", and said British troops were behind it despite lacklustre public support for the cause at home.
"There is not one member of our services, not one man, not one woman among the tremendously courageous professionals that we send out there who questions this cause. Their frustration if anything is that people back here in the United Kingdom don't understand what they are achieving."
Asked whether the war was worth the deaths of 100 British service members, Chief of Defence Staff Sir Jock Stirrup told Sky News:
"The question 'is it worth it' is one that can never really be answered. How much value do you put on a human life?
"All I can say is that Afghanistan is a worthy cause. All I can say is that security in Afghanistan and that whole ungoverned space is very important for the United Kingdom. It is in our national interest And we are succeeding."
