The match in Pamplona was halted for around 30 minutes when the barrier tumbled to the ground following Osasuna's 12th-minute opening goal against Betis and fans spilled on to the pitch.
Around 60 fans were treated on site for minor complaints and 10 were taken to hospital but none suffered serious injury, according to the local Red Cross.
The Spanish professional league (LFP) decided there would be extended halftime breaks in the other three games involving relegation-threatened sides to allow the Osasuna-Betis match to catch up.
The game ended 2-1 to Osasuna but they were condemned by results elsewhere and relegated after 14 seasons in the top flight.
"Despite the victory the results just didn't go our way," Osasuna coach Javi Gracia told a news conference.
"Any analysis of the match and the situation is pointless given the state of the team, the club and everyone else," he added. "It's a very sad day for me personally and professionally."
Valladolid, who were promoted in 2011-12, went down following a 1-0 home defeat by Granada, who along with Almeria and Getafe avoided the drop.
Almeria drew 0-0 at home to fourth-placed Athletic Bilbao and Getafe beat Rayo Vallecano 2-1 in a bad-tempered clash between the two Madrid clubs which ended in a mass brawl.
Betis finished in 20th and last place with 25 points from 38 matches, Valladolid were 19th on 36 and Osasuna 18th on 39, a point behind Almeria and Elche. Getafe finished in 13th with 42 points and Granada in 15th on 41.
Atletico Madrid won their first La Liga title in 18 years on Saturday thanks to a 1-1 draw at second-placed Barcelona in a thrilling climax to the campaign.
Villarreal won 2-1 at Real Sociedad on Sunday to leapfrog the Basque club into sixth, with both qualifying for next season's Europa League along with this term's winners Sevilla.
The Andalusian club finished fifth, four points ahead of Villarreal, after beating Elche 3-1 in Sunday's late kickoff.
(Reporting by Iain Rogers, editing by Ed Osmond)
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