Nico Rosberg, who drove for Williams between 2006-09 but won his 2016 title with Mercedes, joined champion father Keke on a day of memories for a team who once dominated the sport but have had a leaner time of late.
"Frank (Williams) gave me the big breakthrough by taking me into Formula One. It was a great time and I like to come back here and be thankful to the team," said the German, who retired at the end of last year.
"They are the last family-run team and it's a very special connection," said 1982 champion Keke.
Indian Karun Chandhok took Nigel Mansell's title-winning FW14B car out on track for the first time since the 1992 season with the Briton on hand to reminisce.
"I think the most amazing thing is the noise," said Mansell. "The V10, when that fires up properly, oh my goodness. The hairs stand on end. And it reminds us why our hearing is perhaps a little bit impaired."
Some 50,000 fans registered for free tickets, with organisers estimating 30-40,000 turned up.
TRACK RECORD
Williams have not won a race since Pastor Maldonado triumphed in 2012, with the Venezuelan present to catch up with old friends, but their track record is up there with Ferrari and McLaren and still miles ahead of Mercedes.
Only Ferrari have won more than Williams's nine constructors' championships, between 1980 and 1997. They also have seven drivers' titles and 114 race wins.
Frank Williams and co-founder Patrick Head founded Williams Grand Prix engineering in 1977 and ran a March chassis in that season.
Williams, now 75 and only recently out of hospital where he was being treated for pneumonia, made a rare public appearance while Head flew in from Sicily.
"People describe it as the English version of Ferrari but I think it's better than that," said performance engineering head Rob Smedley, who previously worked for the Italian team with Brazilian Felipe Massa.
"I think the British fans are just much more passionate and much more loyal."
Formula One bosses Sean Bratches and Ross Brawn also attended.
"This is representative of what we should be doing collectively as a sport," Bratches told Reuters. "The responsibility doesn’t just fall to Formula One, it falls to the teams, to all the participants."
(Reporting by Alan Baldwin, editing by Ken Ferris)
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