In the first such demonstration since the generals seized power on Monday, activists chanted, "Military dictator, fail, fail; Democracy, win, win" and held banners reading "Against military dictatorship." Bystanders offered them food and water.
Many in the crowd wore red, the color of Suu Kyi's National League for Democracy (NLD) which won Nov. 8 elections in a landslide, a result the generals have refused to recognize claiming fraud.
As the protest swelled and activists issued calls on social media for people to join the march, the country's internet crashed.
Monitoring group NetBlocks Internet Observatory reported a "national-scale internet blackout," saying on Twitter that connectivity had fallen to 54% of ordinary levels. Witnesses reported a shutdown of mobile data services and wifi.