Spain moved closer on Monday to imposing direct rule over Catalonia to thwart its independence push as Madrid's High Court signalled a hardening line by jailing the leaders of two of the largest separatist organizations.

Marking the first time senior figures in the secessionist camp have been imprisoned since Catalonia's Oct. 1 independence referendum, the court ordered the heads of the Catalan National Assembly and the grassroots Omnium movement held without bail pending an investigation for alleged sedition.

Prosecutors allege that the ANC's Jordi Sanchez and Omnium's Jordi Cuixart played central roles in orchestrating pro-independence protests that last month trapped national police inside a Barcelona building and destroyed their vehicles.

Jordi Cuixart, left, leader of Omnium Cultural, and Jordi Sanchez of the Catalan National Assembly arrive at the High Court in Madrid on Monday. (Javier Barbancho/Reuters)

Catalan regional President Carles Puigdemont commented on Twitter: "Spain jails Catalonia's civil society leaders for organizing peaceful demonstrations. Sadly, we have political prisoners again," making an allusion to past military dictatorship.

Spain jails Catalonia's civil society leaders for organising peaceful demonstrations. Sadly, we have political prisoners again —@KRLS

The High Court also banned the Catalan police Chief Josep Luis Trapero from leaving Spain and seized his passport while he is being investigated for alleged sedition over the same incident, though it did not order his arrest.

In a confrontation viewed with mounting alarm in European capitals and financial markets, Puigdemont failed on Monday to respond to Madrid's satisfaction to Prime Minister Mariano Rajoy's ultimatum to clarify whether he had declared unilateral independence in a speech last week.

He now has until Thursday to back down.

In reply, Rajoy said Puigdemont's stance had brought Madrid closer to triggering Article 155 of the constitution, under which it can impose direct rule on any of the country's 17 autonomous communities if they break the law.

A protester bangs a pot to protest against the decision of Spanish High Court to remand in custody the two Catalan separatist leaders, in El Masnou town, north of Barcelona, on Monday. (Albert Gea/Reuters)

Thousands have demonstrated in the Catalan capital Barcelona and other Spanish cities both for and against independence. So far the crisis has been largely violence-free, except for the day of the referendum when national police assaulted voters with batons and rubber bullets in an effort to thwart the ballot.

The ANC, which has organized protests of hundreds of thousands of secessionists in the past, called on Monday after the court ruling for peaceful demonstrations in front of the Spanish government delegation offices in the Catalan cities of Barcelona, Girona, Lleida and Tarragona.

Catalan police chief's passport withdrawn

The High Court ruled that Trapero, the Catalan police chief, would have his passport withdrawn but rejected a request from the state prosecutor for him to be held in custody while the investigation continues.

Trapero is a hero to the secessionists after his force took a much softer stance than national police in enforcing the ban on the referendum. Prosecutors say he failed to order his force to rescue national police trapped inside the Barcelona building.

Josep Lluis Trapero, the head of the Mossos d'Esquadra, the Catalan regional police force, leaves the High Court in Madrid on Monday. (Susana Vera/Reuters)

Judge Carmen Lamela ruled that there was insufficient evidence to detain Trapero, but did not rule out doing so if more evidence became available.

Suggesting that Puigdemont and his team were in no mood to follow Rajoy's game plan, Catalan interior minister Joaquim Forn said Article 155 would not allow Madrid to remove members of the Catalan government.

The article's terms on direct rule, which has never been applied, are vague.

It says Madrid can "adopt any measure" to force a region to meet its constitutional obligations, with the approval of Spain's lower house. The wording suggests this could include anything from taking control of regional police and finances to installing a new governing team or calling a snap election.