With Amsterdam hosting the Olympic Games, in 1928 the Four Days Marches became international, with teams taking part from Germany, Great Britain and Norway. Non-Dutch participation continued, with the first Swiss group taking part in 1933. From the 1920s the numbers of walkers increased significantly. In 1920 under 500 had taken part, compared with 1,155 in 1928. In 1931 numbers exceeded 2,000 for the first time, with over 3,000 in 1935 and over 4,000 in 1937. The increase was mainly due to greater civilian participation, civilians outnumbering servicemen in all marches from 1932.