Texas Senator Ted Cruz was booed as he failed to endorse Donald Trump as the US Republican nominee, during a speech at the party convention in Cleveland.
Mr Cruz only went as far as congratulating the man who had been his bitter rival in the primary contests.
Angry chants of "We want Trump!" and "Endorse Trump!" grew louder as the senator came to the end of his speech.
Indiana Governor Mike Pence then took centre stage to give his vice-presidential acceptance speech.
Mr Pence praised Mr Trump's record as a businessman and said the country had "but one choice" in November.
"It's change versus the status quo, and when Donald Trump is president, the change will be huge," he said.
But Mr Cruz is likely to steal the headlines after stoking the rage of the convention, to such an extent that his wife Heidi had to be escorted from the floor.
Cruz takes a risk: analysis by Anthony Zurcher, BBC News, Cleveland
A former colleague of Ted Cruz's once told me that the one unforgivable sin in the Texas senator's mind is to insult or demean Rafael Cruz, Ted Cruz's father. Donald Trump did that toward the end of the presidential primary campaign, questioning whether the elder Cruz had ties to John Kennedy's assassination. And on Wednesday night, before a national audience on the Republican Party's biggest stage, the Texas senator got his revenge.
As Mr Cruz left the stage after his non-endorsement speech, showered in boos, he gave a brief thumbs-up, apparently satisfied that the deed was done. And what he did was turn the Republican convention on its head once again. Now, everyone will be talking about his speech - overshadowing what should have been vice-presidential nominee Mike Pence's coming-out party.
Mr Pence is the stable, trusted conservative politician who was supposed to rally unsure Republicans behind their new standard-bearer. Instead, Mr Cruz revealed a party still sharply divided. While those inside the arena seemed united in booing the senator, some conservatives outside the Trump-friendly confines praised his actions.
Politically, Mr Cruz has made a risky move, however. He has staked his future presidential hopes on the chance that Republicans will view him as a principled conservative and not a right-wing Judas. Given that Donald Trump received more than 12 million votes in 2016, that is a lot of support that he may have just permanently angered. But an insult was answered. "Lyin' Ted" had his moment of vengeance.
In his speech, Mr Cruz told supporters to "vote [with] their conscience" and not to stay at home for the general election.
He finished second to Mr Trump in the delegate count during a campaign that featured personal attacks by the New York businessman on Mr Cruz's wife and father.
The boos that drowned him out stopped when Mr Trump entered the convention hall before Mr Cruz had finished speaking.
In other developments at the convention on Wednesday:
- Shouts of "Lock her up!" directed at Democrat Hillary Clinton rang out from the floor for the second night running
- Florida Senator Marco Rubio gave a full-throated support of Mr Trump via video
- There were 17 arrests outside the arena after a scuffle broke out from a flag-burning in the streets
- Eric Trump talked effusively about his father
Republican war of words
Trump on Cruz
- "I get along with everybody. You get along with nobody" - speaking in CNN debate 25 February
- "Lyin' Ted Cruz just used a picture of Melania from a GQ shoot in his ad. Be careful, Lyin' Ted, or I will spill the beans on your wife!" - tweet sent 23 March
- "I mean, what was he doing - what was he doing with Lee Harvey Oswald shortly before the death? Before the shooting?" - comment on Ted Cruz's father Rafael and the Kennedy assassination, 3 May
Cruz on Trump
- "The next Republican nominee needs to be able to make that case against Hillary [Clinton]. If Donald tried to do it, Hillary would turn to Donald and say, But gosh, Donald, you gave $100,000 to the Clinton foundation and I went to your wedding" - speaking in CNN debate 25 February
- "This man is a pathological liar. He doesn't know the difference between truth and lies. He lies - practically every word that comes out of his mouth" - speaking 3 May
Mr Trump later said he knew Mr Cruz would not endorse him but had let him speak anyway.
New Jersey Governor Chris Christie said his actions were "awful" and New York Congressman Peter King called the speech "disgraceful".
The drama came at the end of the day when the Republicans tried to finally put the plagiarism scandal behind them.
Melania Trump faced accusations of copying after it was discovered that lines in her remarks matched almost word for word a convention speech delivered by Michelle Obama in 2008.
After initial denials from the campaign, Meredith McIver, a Trump Organisation employee, admitted her role in writing the speech and apologised for the "chaos I have caused".
Thursday night is the main event of the four-day event - Mr Trump's acceptance speech.
The Republican convention - all you need to know
1. What is the point? At a convention, each party formally nominates its candidates for president and vice-president, and the party unveils its party platform, or manifesto.
2. Who is going? There are 2,472 delegates attending, selected at state and congressional district conventions, and representing each US state and territory. Plus 15,000 journalists and thousands of other party grandees, lawmakers and guests.
3. Who isn't going? Some senior figures who dislike Donald Trump have stayed away, including two ex-presidents named Bush, former nominee Mitt Romney and Ohio Governor John Kasich.
4. What is the schedule?
- Thursday - Donald Trump, introduced by daughter Ivanka