Protesters in Barcelona carried a large 100-meter flag, half bearing the Catalan region's colors and the other half those of Spain as loudspeakers played festive music.
There was a strong undercurrent of anger at Mas' secessionist policies. Some shouted, "We won't get fooled, we are Spain," while others wore the Spanish national football team's jersey and chanted, "Barcelona plus Madrid equals La Roja" the squad's popular name.
Maite Avila, 39, who works for Mas' regional government, said she was furious at those leading the independence movement.
"Mas is only able to get away with this because we live in a democracy, otherwise someone would have cut his throat," said Avila, who was waving a Spanish flag.