Reuters television footage showed players arriving for the session which began at 1730 local time (1530 GMT) and Messi did not appear.
IMAGES from the El Clasico match played at Estadio Santiago Bernabeu in Madrid on Sunday.
Croatian forward Ante Budimir scored twice to help Osasuna earn a shock 4-2 home win over leaders Barcelona.
Barcelona manager Luis Enrique believes his team deserves to win "the best league in Europe" as they aim to clinch La Liga in a potentially thrilling climax to the Spanish season on Saturday.
Goals from Memphis Depay, Luuk de Jong and the returning Ansu Fati helped Barcelona claim a much-needed 3-0 home victory over Levante in LaLiga on Sunday.
Barcelona head into Saturday's 'Clasico' against Real Madrid with a whopping 11-point gap over their bitter rivals in the Liga title race -- and they also have the advantage in financial terms, albeit by smaller margins.
Messi was three ahead of nearest rival Kylian Mbappe, who scored 33 times for Paris St Germain as they won France's Ligue 1.
PSG's Champions League tie with Newcastle is like a final, says Luis Enrique
Lionel Messi scored one penalty and missed another as his double helped Barcelona sweep Levante aside in the second half in a 4-1 victory on Sunday to maintain their perfect La Liga start.
A round-up of Saturday's action in LaLiga, Ligue 1 and Serie A.
Sporting marked the final home game of coach Ruben Amorim's reign in stunning fashion as Viktor Gyokeres' hat-trick saw them come from behind to thrash Manchester City 4-1 and maintain their superb start to the Champions League on Tuesday.
Barcelona were hailed as La Liga champions-elect in the Catalan media after their 1-0 win at Real Madrid on Saturday, while the capital's press called for sweeping changes at Real after another domestic failure.
Barcelona coach Luis Enrique said on Friday he trusted his team to handle the occasion of this weekend's season finale at lowly Granada, where a win would guarantee their 24th La Liga title.
The Argentine striker had hosted a team meeting on Monday to kick start preparations for the clash against Atletico Madrid.
"Barcelona and Sergio Busquets signed this morning an agreement to renew the player's contract, which will end on June 30 2023," Barca said in a statement.
Real Madrid showed their character by beating Barcelona 2-0 in Sunday's top-of-the-table 'Clasico', leapfrogging their rivals at the La Liga summit after a week in which their pride had taken a beating at home and in Europe. Brazilian teenager Vinicius Jr scored the opener in the 71st minute with a strike that deflected off Gerard Pique, flummoxing goalkeeper Marc-Andre ter Stegen who had made some jaw-dropping saves, and substitute Mariano added the second in added time.
Bartomeu's lawyers could not immediately be reached for comment.
Barcelona closed in on the LaLiga title as they picked up a 1-0 win over Osasuna and holders Real Madrid went down to
Atletico Madrid won the La Liga title for the first time since 2014
The following is the draw for the Champions League last 16 which was held in Nyon on Monday:
The 33-year-old Messi, who signed his last contract in 2017, has become more outspoken against the club's hierarchy in the last year and radio station Cadena Ser said last Thursday he is unhappy with how Barca is being run and wants to leave in 2021.
Real fought back after a calamitous own goal from keeper Keylor Navas gave Betis the lead in the 25th minute, just after the Costa Rican escaped a possible red card. Cristiano Ronaldo pulled the hosts level in the 41st with a diving header.
According to widespread Spanish media reports, Netherlands coach and former Barca defender Ronald Koeman is set to be named as Setien's successor.
Neymar's acrimonious split from Barcelona escalated on Thursday as the Paris St Germain forward announced plans to sue the La Liga club, two days after Barca revealed they were suing the Brazilian over breach of contract.
Manchester City squandered a three-goal lead to draw 3-3 with Feyenoord in the Champions League on Tuesday.
The numbers behind Saturday's La Liga "Clasico" between Real Madrid and Barcelona at the Bernabeu stadium (1600 GMT).
Lionel Messi kept his nerve from the penalty spot to give Barcelona a 1-0 win over Real Sociedad in La Liga on Saturday and return his side to the top of the table.
When Barca and Neymar were at the peak of their powers in 2017, however, he shook world football by heading to PSG for a record-smashing 222 million euros ($269.89 million).
Lionel Messi was seen smiling as he and his fellow players worked through persistent rain at the club's Joan Gamper training complex under the orders of new coach Ronald Koeman.
Real and Barca have won the title in 15 of the last 16 seasons, with Atletico breaking the duopoly in 2014.
Gijon spoiled Asier Illarramendi's return to Real Sociedad's midfield after rejoining from Real Madrid this week when they held the Basque club, coached by Scot David Moyes, to a 0-0 draw in San Sebastian.
Barcelona's Raphinha scored deep into stoppage time to hand his side a dramatic 5-4 comeback win at Benfica and a place in the Champions League last 16.
Real Madrid got their LaLiga title defence off to a limp start as they were held to a 1-1 draw at Mallorca.
Barcelona are top of La Liga and have lifted back-to-back league titles under the Spanish coach, but there are concerns over the team's levels of performance and style.
IMAGES from the football matches played across Europe on Sunday.
Antoine Griezmann scored two goals and provided two assists as Barcelona staged a stunning last-gasp comeback before beating Granada 5-3
Spain's biggest selling sports daily Marca summed it up in Monday's headline after Barcelona wrapped up a fifth La Liga title in seven years and their 23rd overall on Sunday: 'Messi's League'.
Leaders Barcelona and second-placed Atletico Madrid took full advantage of Real Madrid's stumble on Sunday, Barca winning 4-0 at Levante and Atletico thumping visiting Sevilla by the same scoreline at the Calderon.
The favourites to replace Erik ten Hag, who was sacked as Manchester United manager on Monday.
Barca had the chance to go top after Real drew a fourth consecutive game in all competitions for the first time in over a decade but they were rattled by rampant Celta