Rob Reiner & Wife Michele's Son Nick Reiner Arrested for Murder

11:16 AM PT -- Nick Reiner's original $4 million bond has been changed ... he's now being held without bail.

Rob and Michele Reiner's son Nick has been arrested and is being held on $4 million bail, TMZ has learned.

The 32-year-old is in the custody of the Los Angeles Police Department as of Monday morning, according to online records. He was booked at the LAPD jail.

As we reported ... the couple was found by one of their daughters Sunday with their throats slit inside their Los Angeles home. LAPD Chief Jim McDonnell said Nick is a suspect in the case, and announced his arrest on suspicion of murder.

Rob, Michele and Nick were at Conan O'Brien's Christmas party Saturday night ... at the party, Rob and Nick got into a "very loud argument," loud enough for many others to hear ... Reiner family sources tell TMZ. We're told Rob and Michele then left the party. We do not know if Nick left as well.

Friends and collaborators Billy Crystal and Larry David were seen leaving the Reiner home Sunday evening looking very distraught.

An outpouring of tributes have come in following the tragedy, including those from Barack Obama, Stephen King, and Ben Stiller, among others.

Page Six was first to report the arrest.

Rob was 78. Michele was 68.