Red Bull’s home favourite and title contender Max Verstappen, with tens of thousands of orange-shirted fans packing the grandstands, was second in the morning session and then fifth in the afternoon.
Mercedes’ seven times world champion Lewis Hamilton, who is three points clear of his Dutch rival, was fastest in a red-flagged and disrupted opening session before his car broke down in the second.
“I just lost power and so they told me to stop,” said Hamilton. “It’s not the end of the world. “It’s been a beautiful day, it’s amazing to see the crowd, so many people here.”
The breakdown brought out red flags, with a further stoppage caused by Haas’s Nikita Mazepin going into the gravel.
Leclerc’s fastest lap of one minute 10.902 seconds was 0.154 quicker than Sainz, with Alpine’s Esteban Ocon third.
“It was really enjoyable to see all of them in orange and having fun in the grandstands,” said Verstappen of his home reception.
“There’s a few things we need to understand better to get a bit more pace out of it but it’s also not very representative because my lap was on a used set (of tyres) and the lap I had to abort I think would have been good enough for first.”
More than 35 minutes of the opening hour at the seaside circuit were lost after Sebastian Vettel’s Mercedes-powered Aston Martin suffered a power unit failure and stopped, leaving a trail of oil.
Vettel grabbed a fire extinguisher as smoke came from the back but the car appeared to remain electrically live and could not be removed until it was declared safe by a marshal in protective clothing.
By the time the process was complete, only six minutes remained and a roar went up from the crowd as the cars returned to the track in a final rush.
Hamilton lapped in 1:11.500 with Verstappen 0.097 slower, after losing time in traffic, and Sainz third and 0.101 off the pace.
AlphaTauri’s Yuki Tsunoda had an early spin that ended his involvement.
All three days have been sold out, with 70,000 spectators allowed to attend on each and nearly all supporting Verstappen or his team at an old-school track with some standout banked corners.
The 23-year-old Dutch driver is by far the most successful in his country’s Formula One history, the youngest ever race winner and the first to challenge for the title. He is three points behind Hamilton after 12 races.
A plane was spotted overhead during the session trailing a banner declaring ‘7xWDC. Simply lovely. #TeamLH’ in support of Hamilton.
The Sun newspaper reported that one of the Briton’s fans had paid £1,600 ($2,211) to charter the plane. “It’s a cheeky opportunity to show Lewis some extra support as he steps into the lion’s den here in Max’s back yard,” said fan Neil Steele.