A smoke flare has been set off at The Open and orange powder thrown at a green by Just Stop Oil protesters.
American golfer Billy Horschel helped remove a woman before greenkeepers removed the powder with leaf blowers.
The incident, on the 17th hole of the Royal Liverpool Golf Course in Hoylake, happened at around 12:20 BST.
“A protester was quickly apprehended on the 17th hole and is one of four people who have been arrested,” The R&A, which organises The Open, said.
“Play was not disrupted and we thank marshals, players and spectators for their vigilance.”
Just Stop Oil said in a statement: “Three Just Stop Oil supporters ran on to hole 17. They set off a smoke flare and threw orange powder paint on the green.”
Canada’s Corey Connors, Swede Alex Noren and Horschel all completed the par-three hole in three shots before moving on to their final hole.
It is understood police have now deployed one officer to each green on the course.
Merseyside police said: “Two males and two females were arrested on suspicion of conspiracy to commit criminal damage and public nuisance.
“Merseyside Police respects the right to protest and expression of views but anti-social, criminal behaviour or disorder will not be tolerated and will be dealt with robustly.”
Open organisers were prepared for a protest, putting in extra security precautions and advising players to not engage with anyone who tried to disrupt play.
Just Stop Oil protesters have disrupted several sporting events in England this year, including tennis at Wimbledon and an Ashes cricket Test at Lord’s.
England wicketkeeper Jonny Bairstow carried one pitch invader to the boundary at Lord’s in June, while three people ran on to a court at Wimbledon throwing orange paper and jigsaw pieces earlier this month. There was also a protest in April at snooker’s World Championship, where a man jumped on a table and dropped orange powder.
