Athletic Bilbao fans flooded the streets of their city on Thursday to celebrate the team's Copa del Rey triumph, their first major trophy for 40 years, paraded down the Nervion river on a boat.

Players boarded a barge known as the 'Gabarra' and sailed 13 kilometres down the river from the affluent town of Getxo to the Bilbao city hall, where the party continued.

Hundeds of thousands of fans lining the river banks and crammed onto balconies let off smoke bombs and flares, waving flags and scarves in celebration as Athletic celebrated their first cup win since a domestic double in 1984.

Ernesto Valverde's Athletic side beat Mallorca on penalties last weekend in Seville to win the cup, four decades after the club defeated a Barcelona team led by Diego Maradona in the final.

"At Lezama (the club's youth academy) we have the photo of the Gabarra, and we've always talked about how it would be to live something like this," said midfielder Ander Herrera.

Players from that team, immortalised in photos in most Bilbao bars and restaurants, were also present at the celebrations.

"The whole squad's here and we're remembering old times and enjoying it, so these kids see what these fans are truly about," said Javier Clemente, who coached Athletic between 1981-1986.

"(Athletic) teams are sometimes good, sometimes bad, but the fans are always good. The fans give you goosebumps."

The Athletic players' boat was followed by a fleet of other vessels filled with supporters, including one driven by Barcelona defender Inigo Martinez.

The centre-back, who played for Bilbao between 2018 and 2023, when he joined Barca, did not want to miss the celebrations.

With schools shut early so children could enjoy the celebrations, over a million fans were expected to attend.

Athletic coach Valverde, a keen photographer, boarded the boat with a camera, and took snaps of the joyous scenes.

"This is for those who are here, those who are not here and those who have left us," said Athletic striker Inaki Williams.

"For those who played at Athletic and gave everything for Athletic to be a champion side."

Captain Iker Muniain led the celebrations from the balcony of the city hall.

"Who are we? Athletic! And what are we? Champions!" he yelled to a sea of supporters decked out in red-and-white, who will party long into the night.