The officer in charge of the operation at sea, known only as Constable A, said that from his position at the bridge of a nearby ship the San Pietro and the Ocean Explorer at one point appeared to be touching.
Despite warnings to return to a specific safety zone, already established, the San Pietro continued to move closer to the Ocean Explorer and responded to police instruction with offensive hand gestures, Constable A said.
"My understanding is they were not complying with instructions. They were not heeding to warnings, and there were several warnings given. They were being belligerent, therefore police boarded and arrested the master."
When defence counsel Ron Mansfield asked what motivated Constable A to give authority for officers to board the San Pietro, he said became concerned at a feeling "that something was building" among the protest water craft.
"You could just sense it ... they were almost forming a posse for lack of a better term. The San Pietro deployed buoys and things then the San Pietro moved towards the bow."
When questioned by police prosecutor Sergeant Barry Woon, Constable A said there were fears the situation could escalate.
"I had grave concerns for the safety of the people aboard San Pietro and the equipment and the Ocean Explorer. If something tragic did happen, then police and others would be obliged to help and then put themselves in danger."
A buoy and flag were among a dozen pieces of evidence presented in the courtroom yesterday.
The hearing will continue tomorrow.