Regional president Carles Puigdemont was the first to sign the Declaration of the Representatives of Catalonia document.
Dozens of other politicians signed it after him in the regional parliament in Barcelona.
The signing ceremony came a few hours after Mr Puigdemont addressed the parliament, saying Catalans had earned the right to independence from Spain after a referendum on October 1.
However, he called for dialogue with Spain’s government, which has condemned the referendum as illegal and unconstitutional.
The central government in Spain responded by saying it did not accept the declaration of independence by the separatists and did not consider the October 1 referendum or its results to be valid.
Mr Puigdemont said the landslide victory in the referendum gave his government the grounds to implement its long-held desire to break century-old ties with Spain.
However, he proposed the regional parliament “suspend the effects of the independence declaration to commence a dialogue, not only for reducing tension but for reaching an accord on a solution to go forward with the demands of the Catalan people”.
“We have to listen to the voices that have asked us to give a chance for dialogue with the Spanish state,” Mr Puigdemont said.
Such a move would help reduce political tensions and reach “an accord on a solution to go forward with the demands of the Catalan people”, Mr Puigdemont added.
About two dozen tractors flying secessionist flags paraded near parliament and thousands of separatists gathered in the promenade next to Barcelona’s Arc de Triomf, where the movement’s main grassroots group has called for a rally.
The Spanish government does not accept the “implicit” declaration of independence by the Catalan separatists and the results of a banned referendum cannot be considered valid, an official said.
The official said that the government of Prime Minister Mariano Rajoy cannot accept validating a Catalan referendum law that was suspended by Spain’s Constitution and called the referendum “fraudulent and illegal”.
Mr Rajoy’s government “considers it inadmissible to make an implicit declaration of independence and then leave it in suspension in an explicit manner,” according to the official.
Mr Puigdemont was highly critical of the Spanish government’s response to the referendum and the violent police reaction that left hundreds injured on voting day, but said Catalans have nothing against Spain or Spaniards, and that they want to understand each other better.
“We are not criminals, we are not crazy, we are not pulling off a coup, we are not out of our minds. We are normal people who want to vote,” he said.
Opposition leader Ines Arrimadas of the Ciudadanos (Citizens) party slammed the speech.
“This is a coup. Nobody has recognised the result of the referendum. Nobody in Europe supports what you have just done,” she said.
“Stop saying the Catalan people are united. Above all after what you have done. You have forced us to turn against one another,” she said, addressing Mr Puigdemont during the parliament session.
“The majority of Catalans feel they are Catalans, Spanish and Europe. We won’t let you break our hearts into bits,” Ms Arrimadas said.
Some 2.3 million Catalans - or 43% of the electorate in the region - voted in the referendum.
Regional authorities say 90% of those who voted were in favour of independence.