WASHINGTON: A judge, called a ‘hater’ by Donald Trump for his handling of a lawsuit related to the businessman's Trump University real estate school, has unsealed documents related to the case.

Trump, the presumptive Republican presidential nominee for the Nov 8 election, is fighting a lawsuit that accuses his school venture of misleading thousands of people who paid up to $35,000 for seminars to learn about the billionaire's real estate investment strategies.

In an order signed on Friday, United States (US) District Judge Gonzalo Curiel said that materials, including Trump University procedures on dealing with students and the media, should be unsealed.

He noted they had already been published by the media organisation Politico and that a magistrate judge described them previously as ‘routine’ and ‘commonplace.’

At a rally in San Diego on Friday, Trump criticised Curiel for his handling of the Trump University case.

“I have a judge who is a hater of Donald Trump. A hater. He's a hater,” Trump said.

“We're in front of a very hostile judge. The judge was appointed by Barack Obama,” Trump said, adding he believed Curiel was Mexican.

Curiel is an American who was born in East Chicago, Indiana, and graduated from the Indiana University School of Law.

Legal scholars said Trump could face consequences for slamming the judge, although many speculated that Curiel was unlikely to sanction him formally.

“Mr. Trump's conduct could be subject to sanction for indirect criminal contempt of court,” said Charles Geyh, a legal ethics expert at the Indiana University Maurer School of Law.

“He has impugned the honesty of the judge in a pending case, and has done so in the context of a political rally that seems calculated to intimidate by inciting anger among his supporters,” he said.

Arthur Hellman, an expert on federal courts and judicial ethics at the University of Pittsburgh School of Law, said the judge was in a difficult position.

“He can't respond directly. He's not supposed to talk out of court about proceedings before him. Judges have gotten into trouble defending themselves from attacks. The judge's hands are really tied,” he said.

Trump has drawn criticism for his comments about immigrants from Mexico, some of whom he has said were criminals and rapists.

He has proposed building a wall along the US border with Mexico to prevent illegal immigration and requiring Mexico to pay for it.

Hispanics are a critical voting bloc in US presidential elections.

Last week, Trump knocked one of the highest-profile Hispanic women in the Republican Party, criticising New Mexico Governor Susana Martinez for her handling of the state's economy.

Her office said his criticisms were not substantive. Martinez has been touted as a potential vice presidential pick for a Republican ticket.