A judge has dismissed a lesser murder charge against former Minneapolis police officer Derek Chauvin, but he will still face the most serious murder charge in George Floyd’s death.

According to a ruling released on Thursday, Hennepin County Judge Peter Cahill said prosecutors did not have probable cause to charge Mr Chauvin with third-degree murder, but said the evidence supported taking a second-degree murder charge to trial.

If convicted of second-degree murder, Mr Chauvin could spend decades in prison.

Minnesota Governor Tim Walz announced he had activated the Minnesota National Guard as a “precautionary step” following the ruling, which he said marked “a positive step in the path toward justice for George Floyd”.

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The fatal arrest of Mr Floyd on May 25, captured on video by bystanders, sparked angry and sometimes violent protests in Minneapolis and elsewhere over police brutality and racism.

Mr Chauvin is white, while Mr Floyd was a black man.

Judge Cahill described in detail how Mr Chauvin used his knee to press Floyd’s neck and face “into the unforgiving concrete of Chicago Avenue” in Minneapolis even after Mr Floyd pleaded that he could not breathe and then became unresponsive.

Judge Cahill upheld a manslaughter charge against Mr Chauvin and all six charges against three other officers – Thomas Lane, J Alexander Kueng and Tou Thao – for allegedly aiding and abetting Mr Chauvin in the murder.

Mr Kueng and Mr Lane helped hold Mr Floyd down by restraining his back and legs, Judge Cahill noted, while Mr Thao kept a group of bystanders at bay.

Earlier this month, Judge Cahill ruled that Mr Chauvin, who had posted a $US1 million ($A1.4 million) bond with a condition he stay in Minnesota, could leave the state as he awaited trial due to safety concerns.