Three women and a man have been arrested at Prestwick Airport after breaking through security fencing and getting onto the main runway.
Prestwick has become a focus for protests over cargo flights
The four say they were investigating reports that Prestwick was still being used as a refuelling stop for US flights carrying bombs to Israel.
A spokesman for the four protesters said they described themselves as "citizen weapons inspectors".
Police said they were arrested just before 0100 BST on Sunday.
There has been an outcry over the use of Prestwick as a stopover for so-called "bomb flights".
The group arrived at the airport late on Saturday night following reports that planes carrying bombs to Israel were refuelling there.
A spokeswoman at the Foreign Office said they could not confirm details of any flights, but added that these flights would be legal if they complied with UK regulations.
A Prestwick Airport spokesman said no flights containing weapons had landed at the airport.
It was reported last week that such flights would only use military bases in the future.