Atletico Madrid remain level on points with Barca thanks to two goals in the final two minutes from Angel Correa and Antoine Griezmann to emerge 2-0 victors at Rayo Vallecano.
And Cristiano Ronaldo scored twice and missed a penalty as Real Madrid eased the pressure on under-fire coach Rafael Benitez with a 3-1 win over Real Sociedad to stay two points behind the leaders.
"It has been a spectacular year and we wanted to finish it this way," said Messi.
"It will be difficult to improve on what we have done, but we always try. We have a great squad that are desperate to continue winning."
Yet, on a day dominated by controversial refereeing decisions, they were handed a gift to open the scoring after a decent start by Betis.
Antonio Adan clearly played the ball before catching Messi as he punched the ball to safety, but referee Inaki Vicando Garrido surprised everyone by pointing to the spot.
From Betis' misfortune the floodgates opened as a lovely one-two between Neymar and Messi allowed the later to tap home his 425th goal for the club.
Luis Suarez added a third with a cool finish from Sergio Busquets's pass inside the first minute of the second-half.
And the Uruguayan rounded off the scoring seven minutes from time as Barca set a new record tally for the calendar year of 180 goals.
At the Santiago Bernabeu, Ronaldo recovered from a terrible start after blasting his first penalty of the day over the bar to put Madrid in front from another spot-kick just before half-time.
"I congratulate the players for their attitude from start to finish. We worked hard, reacted well and got the three points."
Rumours over Benitez's future refuse to go away after a disappointing first six months in charge at the Bernabeu with the former Liverpool boss even claiming there is a media conspiracy against him on Tuesday.
- Controversial penalties -
===========================
But his side had luck on their side in the first-half as Sociedad's lost top-scorer Imanol Agirretxe and playmaker Sergio Canales to injury and saw referee Jose Luis Gonzalez award Madrid two controversial penalties.
The World Player of the Year made amends three minutes before half-time when he converted his second penalty after Yuri was again penalised for blocking Gareth Bale's cross with an outstretched hand.
Despite the adversity they faced, Sociedad kept the ball off Madrid for long spells and hauled themselves level in spectacular style four minutes after the break when Bruma curled home his first league goal for the club into the top corner.
And Vazquez ensured all three points when he coolly controlled and finished Bale's fine cross to make it 3-1 three minutes from time.
Atletico looked to be heading for a second consecutive disappointing result after losing 1-0 at Malaga before the 10-day winter break as Rayo's defence - fresh from conceding 10 to Real Madrid last time out - stood firm for 88 minutes at Vallecas.
However, Diego Simeone's substitutions swung the game in Atletico's favour as Thomas Partey teed up Correa to smash home his first goal in three months.
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