After the first over on Wednesday bowled by England’s James Anderson to Australia batsman David Warner, two men emerged from the stands and moved towards the square, releasing the powder before being tackled by security staff.
England wicketkeeper Jonny Bairstow carried one of the men off the field before ground staff cleared the pitch and play resumed after a delay of around five minutes.
Three people have since been arrested, police said.
Writing on Twitter, a spokesperson for the Metropolitan Police said: “We are aware of protesters on the Lord’s Cricket Ground pitch today, Wednesday, 28 June.
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Police have arrested three people and taken them into custody.”
Just Stop Oil protesters have disrupted other sporting events in England this year, including the Premiership Rugby final at Twickenham and the World Snooker Championship in Sheffield.