The anti-capitalist Popular Unity Candidacy (CUP) party denounced Madrid's plans as the “biggest aggression” against the Catalan people since the Franco dictatorship.
“This aggression will receive a response in the form of massive civil disobedience,” said the party, a key regional power broker, in a statement.
The CUP's threat upped the stakes in the standoff over Catalan independence, which has raised fears of unrest in Spain's deepest political crisis in decades.
Half a million angry separatists took to the streets of Barcelona on Saturday after Spanish Prime Minister Mariano Rajoy announced he would remove Catalan leader Carles Puigdemont and his executive.
Rajoy said Madrid would take control of ministries under unprecedented measures to stop the region breaking away.
The CUP is a key ally of the ruling separatist coalition in the regional parliament, which as a minority government relies on their votes to pass legislation.
It said the details of the protest actions it is planning would be unveiled later this week.
Catalan parties are meeting on Monday to set a date and agenda for a gathering of the regional parliament to debate their next steps. The governing coalition has asked for a session to be held on Thursday, one day before Spain's Senate gathers to approve Madrid's measures.
The CUP favours a faster route to independence than the coalition, and has pressed Puigdemont to lift the “suspension” he placed on a declaration of independence.
Catalan firefighters hinted they may offer resistance in the dispute by refusing to obey orders from national authorities.
“It depends on what they ask us to do. If there is a road that is blocked and they send us to unblock it, maybe we won't go,” said a spokesman for a firefighter association associated to the separatist movement.
Teachers called a protest march for Thursday, and students said they will go on strike from that day.
Half a million angry separatists took to the streets of Barcelona on Saturday after Rajoy announced he would replace Puigdemont and his executive.
To do so, Madrid will use previously untested constitutional powers to stop Catalonia breaking away.