Four farms have been given the go-ahead for trials
Police arrested two people who had apparently chained themselves to the tractor carrying out the work at the farm at Munlochy on the Black Isle, near Inverness.
Four more people tried to block the tractor and their arrests followed five more made earlier on Thursday.
Police said the five protesters arrested in the afternoon had been charged with a breach of the peace after allegedly lying in front of the tractor.
Up to 30 people were involved in the protests over the trials of GM oilseed rape at Roskil Farm.
Police said the people arrested were members of the Highlands and Islands GM Concern group, which has raised fears about the effects of a herbicide to be used in the trials.
The Black Isle site is one of four in the north of Scotland which the Scottish Executive this week gave approval for the farm trials of GM crops.
Officials said planting would take place this autumn as part of a UK-wide farm scale evaluation programme.
Dr Kenny Taylor, chairman of Highlands and Islands GM Concern, said the protests were a result of frustrations boiling over.
Biodiversity
He said: "It is a reflection of the sheer frustration that people in the area feel.
"Despite polls which show that the vast majority of people in the area don't want the crop trials in their area, it has still been approved by the Scottish Executive."
He added: "We sincerely hope that there will be no further arrests and sincerely hope that Ross Finnie (rural affairs minister) will listen to the concerns of people."
The group is seeking legal advice on ways of blocking the planned trials on the Black Isle.
Ministers have said that no GM crops will be commercially grown in Scotland if the results of the programme suggest that they could pose a threat to biodiversity.
Judicial review
However, trials in the UK have been subject to opposition from environmental activists with some crops damaged or destroyed.
Last month, a survey by Highland Council reported that a majority of people were opposed to GM trials.
And environmental campaigners are hoping to win a judicial review of the way ministers handled trials at the Black Isle farm.
Highlands and Islands GM Concern is seeking advice on the range of legal options available.
But Rural Development Minister Ross Finnie said he is "quite satisfied" that the executive is operating within the law.