Police in Aberdeenshire were "understandably" seen as biased in favour of the New York property tycoon, Donald Trump, and against local residents, an internal police investigation seen by the Sunday Herald has concluded.

Remaining neutral in the prolonged, bitter row over Trump's plans for a golf resort at Menie on the northeast coast was "an impossibly hard task" because the majority of calls came from Trump's staff, the investigation report said.

Residents and campaigners accused police of acting as Trump's "private security force" instead of protecting local people's interests. But Trump's spokesman called the accusations pathetic.

Police Scotland has released 26 pages of correspondence and reports about fiercely disputed events at Menie three years ago. This follows an investigation by the Scottish Information Commissioner, Rosemary Agnew.

Officers were present on August 3, 2010 when Trump's representatives used diggers to move a boat and equipment belonging to Michael Forbes, one of the billionaire's ­highest-profile local opponents. They then watched as a fence was erected on what Forbes claimed was his land.

Officers who attended reported it was their understanding that Trump was "entitled to erect the fence in that location". They said they gave suitable advice to that effect.

But the incident prompted several complaints police had backed a "land grab" by Trump International Golf Links Scotland (TIGLS). In response to one, local police inspector Steve Pratt described the land dispute as "very much a civil matter".

He said the decision to deploy officers had not been taken lightly. "I recognise the perception could well be that their very ­presence endorsed the activity that was taking place that day," he said. "That was not my intention."

An internal investigation into the complaints, marked restricted and mostly blacked out, goes further. The "perception of police attendance at Menie estate that day was understandably that police were there to support TIGLS," it concluded.

"It is an impossibly hard task for Grampian Police to appear neutral in every possible scenario at the Menie estate. The majority of our calls will be from TIGLS and as such we will have to work very hard to ensure that we are not seen as being biased towards them," it added.

Forbes told the Sunday Herald he had shown the police his title deeds proving that he owned the disputed piece of land, but they "weren't interested". His property had been damaged when it was moved, he said.

The police should have prevented what happened, Forbes argued. "They were there for Trump, they weren't there for me," he said.

"They don't seem to be interested in poor people, just people with money like Trump. They acted as a private security force for him."

The police documents were obtained by the Montrose filmmaker, Anthony Baxter, but only after he appealed to the information commissioner. His 2011 documentary, You've Been Trumped, showed Forbes and others in a "David and Goliath" battle with Trump.

"The documents prove beyond a shadow of a doubt what residents on the Menie estate have been saying all along," Baxter said. "Grampian Police saw it as their job to side with Mr Trump in his disputes with local residents, to act, in effect, as his security force." Baxter and his producer, Richard Phinney, were arrested and imprisoned by police while filming at Menie in July 2010. All charges were subsequently dropped.

"What is most depressing is that the residents who have had to put up with this police bias continue to face harassment," said Baxter. He accused the police of attempting a "blatant cover-up" by refusing to release the documents until forced to do so by the information commissioner.

He demanded to know whether the police would be held to account for their behaviour. "This is just another example of why Scotland desperately needs a robust independent police complaints commission, as there is in England and Wales," he said.

Police Scotland said officers commonly had to deal with disputes. "The nature and complexities of such situations vary greatly and the development of the Menie estate is no different," said a spokeswoman.

"It is always unfortunate that disagreements, which commence as civil matters, can deteriorate into a situation where police intervention is required," she added, insisting Police Scotland was completely neutral.

On accusations of police bias, the Trump Organisation's executive vice-president in New York, George Sorial, said: "This is absolutely pathetic, untrue and completely without merit."