The central government on Friday reiterated its conviction that Catalonia’s plans to hold a referendum on self-rule would be in breach of the Constitution, and pledged to reply to a letter sent by Catalan premier Artur Mas on the matter within 48 hours.

Deputy Prime Minister Soraya Sáenz de Santamaría offered the Catalan government “dialogue,” but only within the terms allowed by the Constitution.

In his letter to Prime Minister Mariano Rajoy, Mas suggested five legal ways that his government believes could be availed to hold a public vote.

Sáenz de Santamaría was speaking after Catalans celebrated their Diada national day on Wednesday by forming a massive human chain that stretched across the region, in a bid to push demands to be allowed to vote on their future within Spain.

Referring to the Diada protests, Sáenz de Santamaría said she “respected” the right of people to express themselves in this way. On Thursday, Foreign Minister José Manuel García-Margallo described the demonstration as a “success,” adding that the government should investigate what led to the push for independence.

The leader of the main opposition Socialist Party (PSOE), Alfredo Pérez Rubalcaba, on Friday proposed a reform of the Constitution in order to take on board some of Catalonia’s wishes in a way that would see it remain part of Spain. But Sáenz de Santamaría said that “the minimum consensus needed to reform the Constitution” did not exist.

The leader of the Catalan branch of the Socialists, Pere Navarro, on Friday called on the ruling Popular Party and the PSOE to negotiate a “new status” and funding system for Catalonia.