Demonstrators blocked a highway in Arizona as Donald Trump prepared to stage a rally near Phoenix.
The front-runner in the race to be the Republicans' US presidential candidate addressed supporters in Fountain Hills.
Some 50 protesters caused traffic jams as they held up posters with slogans such as "Dump Trump".
The billionaire's rallies have been marred by a series of recent incidents. A meeting in Chicago a week ago was cancelled after protests.
Hundreds of demonstrators had gathered at the University of Illinois and fighting broke out between supporters and protesters in the auditorium where Mr Trump was due to speak.
