Prosecutors were expected to outline evidence they say proves
Bryant sexually assaulted a 19-year-old female employee at a resort June 30.
The 25-year-old Bryant has said he and the woman had consensual sex. He faces a possible prison term of four years to life if convicted.
Legal experts had expected the defense team to waive the hearing and head straight to trial rather than allow prosecutors to lay out their case publicly for the first time.
"The only reason the defense would choose to go ahead with a preliminary hearing when it doesn't have to is it believes given the minimal amount of evidence the prosecution is going to be putting on it may gain more by cross-examining those witnesses," said Stan Goldman, a professor at the Loyola Law School in Los Angeles.
He suggested the defense may call witnesses to testify, a list that could include Bryant himself.
Judge Frederick Gannett had already rejected defense requests to have the woman testify in person and to see her medical records.
Bryant's attorneys huddled with Gannett minutes before the hearing, then began the proceeding. It was the first time details of the case would be revealed in court. The judge has issued a gag order.
Bryant, free on $25,000 bond, left the Los Angeles Lakers' training camp in Hawaii on Wednesday to travel to Colorado.
Security for the hearing was beefed up after dozens of threats were made against the prosecutor, the judge and Bryant's accuser. Judge Gannett has acknowledged receiving letters containing death threats, and two men have been charged with threatening Bryant's accuser.
Bryant landed in a private jet at the Eagle airport and rode to the courthouse in the backseat of a white SUV, reports CBS News' Lee Frank. He got out, offered a helping hand to defense attorney Pamela Mackey and walked to the courthouse doors ignoring shouts from reporters and spectators.
Bryant had to take off a necklace and was checked with a metal-detecting security wand before walking through a metal detector and into the courtroom.
The case against Bryant could lead to a celebrity trial the likes of which have not been seen since O.J. Simpson was acquitted of murder charges eight years ago.
Since Monday, about 300 television, print and radio reporters and camera crews have been arriving in Eagle, filling motel rooms and parking TV satellite trucks in a vacant lot across from the courthouse that normally is a lumber dealer's back yard.
A handful of people lined up at the courthouse to get passes for the hearing. Among them was George Zinn of Salt Lake City, who arrived on a Greyhound bus to watch the spectacle.
"I don't consider Kobe a role model," he said.
Nearby, a group of University of Colorado students handed out packages of condoms and legal contracts that both parties would sign to agree to consensual sex.
At Bryant's initial court appearance on Aug. 6, he said just two words: "No, sir," when Gannett asked if he objected to giving up his right to have a preliminary hearing within 30 days. Unlike that appearance, cameras were banned from the courtroom this time.
Legal analysts had predicted defense attorneys would waive Thursday's hearing because they have no chance of winning it and little chance of learning more than they already know. They also would avoid the disclosure of details of the alleged sexual assault until trial.
"From a public relations standpoint, that type of evidence would most likely be very damaging to Kobe Bryant's reputation," said Karen Steinhauser, a former prosecutor and a professor at the University of Denver law school.
Bryant has the right to go to trial within six months, but he could agree to push that back until later, perhaps after the NBA season ends early next summer.