Republican Senator Rand Paul has declared his candidacy for US president, kicking off the race with a rally in his home state of Kentucky.
"I am running for president to return our country to the principles of liberty and limited government," Paul's website quoted him as saying on Tuesday.
The 55-year-old, labelled ultraconservative by the US media, begins the 2016 race as the second fully declared candidate, behind Senator Ted Cruz of Texas, but he could face as many as 20 rivals for the Republican nomination before the primary process starts in January.
Two other Republicans considered early front-runners, former Florida Governor Jeb Bush and Wisconsin Governor Scott Walker, are expected to soon enter the race.
Whoever becomes the Republican nominee is widely expected to face Hillary Rodham Clinton, the heavy favourite for the Democratic nomination, in the general election. The former secretary of state is expected to announce her candidacy in the next two weeks.
Paul's father, former US Representative Ron Paul of Texas, unsuccessfully sought the Republican nomination several times, appealing to libertarians who favour limited government and oppose tax increases.
He is a frequent contrarian against his party's orthodoxy, questioning the size of the US military and proposing relaxation of some drug laws that imprison offenders at a high cost to taxpayers. He also challenges Republicans' support for surveillance programmes, drone policies and sanctions on Iran and Cuba.
Tech savvy and youth-focused, Paul is expected to be an internet juggernaut his competitors will be forced to chase.
After his speech, Paul was set to answer questions from voters on his Facebook page.