Republican US presidential nominee Donald Trump formally accepts the nomination at the Republican National Convention in Cleveland, Ohio, US July 21, 2016. [Photo/Agencies]

The New York businessman, who has never held elective office, needs 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 unfavorable 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.

Citing a wave of violence in American cities, Trump vowed to restore law and order, warning that 180,000 illegal immigrants with criminal records "are tonight roaming free to threaten peaceful citizens."

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.

"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 Jan 20, 2017, safety will be restored," Trump said. The next president takes office on Jan 20.