Carles Puigdemont -- the former leader of Catalonia who fled Spain over his role in a failed 2017 independence bid for the wealthy region -- returned to Spain on Thursday to address supporters before vanishing despite an extensive police deployment to arrest him.
Police have set up roadblocks in Barcelona and other parts of Catalonia to try to find Puigdemont, who remains the target of an arrest warrant.
Earlier on Thursday, Puigdemont delivered a short speech to thousands gathered near the Catalan regional parliament, which is set to elect a new leader later in the day.
"Long live a free Catalonia!" he shouted when he climbed onto the stage in Barcelona, under the watch of dozens of police officers who made no attempt to detain him.
"I have come here to remind you that we are still here," he said as many in the crowd waved red, yellow and blue Catalan independence flags.
He then disappeared.
Puigdemont, 61, had been expected to try to enter the parliament building for the investiture vote, which went ahead without him.
Catalonia's regional police force said it had arrested one of its officers on suspicion of helping fugitive Puigdemont flee.
The officer allegedly owns a car in which Puigdemont left the scene, according to Spanish media reports.
Around 3,500 people had turned out to hear Puigdemont speak, according to Barcelona city hall.
"He is a very noble person," said Nuria Pujol, a woman in her fifties who come from the Alt Penedes region for the event.
"(He's) the only one who believes in independence and has not stopped believing," she told AFP.
- 'Problem with democracy' -
Puigdemont's dramatic return came just days after Spain's governing Socialists struck a deal with moderate Catalan separatist party ERC to make the Socialist candidate, Salvador Illa, the next head of the Catalan regional government.
ERC is a political rival of Puigdemont's more hardline separatist JxCAT party.
The Socialists won the most seats in the Catalan regional election in May but failed to get a majority and the support of the ERC is crucial.
If a new Catalan regional government is not formed by August 26, a fresh election will be held in October.
Puigdemont led the regional government of Catalonia in 2017, when it pushed ahead with an independence referendum despite a court ban, followed by a short-lived declaration of independence.
He fled Spain shortly after the independence bid to avoid prosecution and has since lived in Belgium and more recently France.
While Spain's parliament in May passed an amnesty law for those involved in the botched secession bid, the Supreme Court ruled on July 1 that the measure would not fully apply to Puigdemont.
"A country that has an amnesty law and does not apply it, has a problem with democracy," he said in his speech.
The head of Spain's main opposition Popular Party, Alberto Nunez Feijoo, called Puigdemont's return and then disappearance "an unbearable humiliation".
"It is unforgivable to damage Spain's image like this," he added.
- Path to independence? -
Spanish Prime Minister Pedro Sanchez agreed to the amnesty law in exchange for JxCAT's crucial support in the national parliament for his fragile minority government.
The move sparked huge street protests that were organised by the right wing.
Sanchez is now facing opposition from parts of his own Socialist party as well as the right over a proposal to give Catalonia full control of the taxes collected in the region.
The measure was promised to the ERC in exchange for the party's support for Illa in Thursday's Catalan investiture vote.
The proposal has for decades been one of the main demands of Catalan independence parties but critics argue it would deprive the central state of a substantial source of revenue.
It must still be approved by Spain's national parliament.
If Illa passes Thursday's investiture vote, he will be the first head of Catalonia's regional government since 2010 who does not come from the separatist camp.
The former health minister has defended the tax agreement made with the ERC, saying it was "favourable for all Catalans".
But former Socialist deputy prime minister Alfonso Guerra has said the tax agreement opens "a path towards a federal system and the independence of Catalonia".
By Alfons Luna