Christian Solidarity Worldwide (CSW) said four Iranian Christians, including a pastor, were in a court hearing after being charged with "action against national security".
If found guilty, the men could face a maximum of six years in prison.
The men are members of the Church of Iran denomination in Rasht and were arrested in a series of raids on Christian homes.
Three of the four men are waiting on a decision after appealing a sentence of 80 lashes for drinking wine during a Communion service.
A judge, who CSW said wasn't presiding over the case and is known for issuing harsh sentences, entered the court room during the proceedings and announced that "Christians make foolish claims".
The charity added that Judge Abolghasem Salavati is known for giving unfair treatment to religious minorities and ordering that defendants be lashed and executed.
CSW said: "His unexpected intervention may indicate that the presiding judge is under pressure from the Secret Police to pronounce a pre-determined verdict and deliver a harsh sentence."
CSW's Chief Executive Mervyn Thomas said the judge's intervention was clearly meant for intimidation.
He added: "Also worrying is the fact that a derogatory statement about a religion that is recognized in the national constitution was uttered in open court by someone who is responsible for dispensing justice.
"We reiterate that the national security charges levelled against these men are spurious and an indication that the authorities persist in criminalising the Christian community for exercising their right to freedom of religion or belief."
The charity has urged the Iranian government to dismiss the charges against them, and to cancel the outstanding charges against the men.
He continued: "The international community must press the Government of Iran to uphold its obligations to respect the right of all of its citizens to freedom of religion or belief."
