Donald Trump accused Democratic rival Hillary Clinton of a legacy of "death, destruction, terrorism and weakness" as US secretary of state and vowed to be tough on crime and illegal immigrants in a speech accepting the Republican presidential nomination.

Trump's 75-minute speech was designed to set the tone for the general election campaign against Clinton, an answer to Republicans who say the best way he can unify the divided party is to detail why the Democrat should not be elected on November 8.

As the crowd chanted: "Lock her up" for her handling of US foreign policy, Trump waved them off and said: "Let's defeat her in November." Thousands of supporters who were gathered in the convention hall roared their approval.

When it was over, Trump was joined on stage by family members as balloons cascaded from above and confetti blew around the arena.

A CNN snap poll of viewers of the speech said 57 percent had a "very positive reaction" to the address and 18 percent a somewhat positive reaction, while 24 percent said it had a negative effect.

Social media sentiment toward Trump based on tweets that mentioned his name was slightly more negative than positive shortly after his speech.

The acceptance speech by Trump, 70, closed out a four-day convention that underscored his struggle to heal fissures in the Republican Party over his anti-illegal-immigrant rhetoric and concerns about his temperament. The event was boycotted by many big-name establishment Republicans, such as 2012 nominee Mitt Romney and members of the Bush family that gave the party its last two presidents.

Trump presented a bleak view of America under siege from illegal immigrants, threatened by Islamic State militants, hindered by crumbling infrastructure and weakened by unfair trade deals and race-related violence.

Accusing illegal immigrants of taking jobs from American citizens and committing crimes, Trump vowed to build a "great border wall" against the border-crossers.

"We will stop it," Trump said.

Trump took positions in conflict with traditional Republican policies. He said he would avoid multinational trade deals but instead pursue agreements with individual countries. He would renegotiate the Nafta trade accord linking the US, Canada and Mexico. He would penalise companies that outsource jobs and then export their foreign-made products back into the US.

"We will never sign bad trade deals," Trump thundered. "America first!"

The New York businessman, who has never held elected office, filled his speech with some of the bravado he used to win the Republican nomination over 16 rivals, punctuating his rhetorical points by waving an index finger.

"I have joined the political arena so that the powerful can no longer beat up on people that cannot defend themselves," Trump said. "Nobody knows the system better than me, which is why I alone can fix it."

In his speech, Trump portrayed himself as a fresh alternative to traditional politicians, willing to consider new approaches to vexing problems and help working-class people who may feel abandoned.

Laying out his case against Clinton, he denounced nation-building policies that were actually put in place to some extent by George W Bush, without mentioning by name the Republican president who launched wars in Afghanistan and Iraq.

Trump said policies pursued by Clinton in Iraq, Libya, Egypt and Syria had made a bad situation worse. He blamed her for the rise of Islamic State militants and blasted her willingness to accept thousands of Syrian refugees.

"After 15 years of wars in the Middle East, after trillions of dollars spent and thousands of lives lost, the situation is worse than it has ever been before. This is the legacy of Hillary Clinton: death, destruction, terrorism and weakness," Trump said.

Clinton senior adviser John Podesta dismissed the speech as painting "a dark picture of an America in decline" and called it a reminder that Trump "is temperamentally unfit and totally unqualified to be president of the United States."

John Weaver, a senior adviser to Ohio Republican Governor John Kasich, a former presidential rival to Trump, said in a tweet that Trump had delivered the "saddest, darkest, most depressing acceptance speech in modern history."

'THINGS HAVE TO CHANGE'

Trump needed a strong performance on Thursday night to improve his chances of getting a boost in opinion polls as Democrats prepare for their own, more scripted convention next week in Philadelphia.

In a contest that pits two politicians viewed as unfavourable by large segments of the American people, Trump also accused Clinton, 68, of being the puppet of big business, elite media and major donors who want to preserve the current political system.

"That is why Hillary Clinton's message is that things will never change. My message is that things have to change - and they have to change right now," Trump said.

Trump said he would speedily address the violence that has dominated headlines, such as the shooting deaths of five Dallas police officers earlier this month. He vowed to defeat "the barbarians of ISIS," the acronym for Islamic State.

"I have a message for all of you: The crime and violence that today afflicts our nation will soon come to an end. Beginning on January 20, 2017, safety will be restored," Trump said. The next president takes office on January 20.

CONVENTION DISCORD

The prevailing narrative at the Cleveland convention has not been about Trump's positions, but dominated instead by the failure of he party's various factions to unite behind Trump.

A series of distractions at the convention largely thwarted a bid by the Trump campaign to show him as a caring father and magnanimous business leader who would bring greater prosperity and safety to the United States.

But in the end, many of these points were made when Ivanka Trump, Trump's daughter, introduced her father.

"I have seen him fight for his family. I have seen him fight for his employees. I have seen him fight for his company and now I am seeing him fight for our country," she said.

Trump's text of his speech, released by his campaign, included extensive footnotes to show where the material originated.

That was perhaps in reaction to the speech given on Monday night by Trump's wife Melania, who was accused of plagiarism when she repeated lines from a 2008 speech by Michelle Obama, Obama's wife.

A staff writer for the Trump Organization later took responsibility for the misstep. - Reuters