The demonstrators have flooded a main avenue, holding up candles, pro-independence "estelada" flags and chanted "political prisoners, freedom".
Cuixart and Sanchez have been detained as figureheads of the separatist movement, with the pair being held without bail while they are investigated for sedition.
A Catalan official says the jailing of two prominent Catalan separatist activists shows how the central Spanish authorities in Madrid are choosing "repression over dialogue".
Spain's national government has demanded that Cataluña's regional leaders clarify a declaration of independence and renounce it.
The court temporarily suspended the law after it was passed by Catalonia's regional parliament on September 6 while judges considered an appeal against it filed by Spain's central government.
2 sailors die 4 days apart in same Kingsland home
Jarrell told his wife he wasn't feeling well while the family was at Disney World last Wednesday for his daughter's birthday. Kingsland Police Department is investigating the case with the United States Naval Criminal Investigative Service.
Regional leaders then went on to stage the October 1 referendum on whether the region should separate from Spain.
In its ruling on Tuesday, the court said the law was against national sovereignty and the "indissoluble unity of the Spanish nation".
"The "right to self-determination" does not exist for any of the "peoples of Spain", it said, adding that the "right" to "promote and enact the unilateral secession" of a part of the country is not recognised in the Spanish constitution.
Meanwhile, Agusti Alcoberro, the man who standing in for Sanchez as head of the Assemblea Nacional Catalana, said peaceful protests would be the local response to what he said was the Spanish government's heavy-handed approach. The protest is against Monday's arrest of two activists in the struggle for independence of Catalonia, AFP reports.
Thousands of supporters, carrying posters reading "Freedom for the political prisoners" banged pots and pans and honked vehicle horns in Barcelona following the jailing Monday.