Francois Fillon has won France's first-ever conservative presidential primary after promising drastic free-market reforms and a crackdown on immigration and Islamic extremism.
He beat more moderate rival Alain Juppe, who had warned of encroaching populism.
"President! President!" chanted the former prime minister's supporters as he declared victory over Mr Juppe in a nationwide run-off election.
Polls suggest 62-year-old Mr Fillon, prime minister from 2007 to 2012 under ex-president Nicolas Sarkozy, would have a good chance of winning the French presidency in the April-May election.
Mr Fillon campaigned on promises of slashing public spending, capping immigration, support for traditional family values and friendlier ties with Russian president Vladimir Putin.
Mr Fillon enjoyed a surprise surge in popularity in recent weeks over long-time front-runner Mr Juppe, who also previously served as the country's prime minister.
In a sober victory speech, Mr Fillon promised to defend "French values" and said France needs "a complete change of software".
"There is in our country an immense need for respect and pride. There is also a call for the authority of the state, and exemplary behaviour by those who lead it," he said.
Mr Fillon, whom Mr Juppe had accused of running a divisive campaign that catered to the far right, struck a slightly more inclusive tone on Sunday night.
"No one should feel excluded from a society that I want to see more just and with more solidarity," he said.
Mr Juppe, 71, congratulated Mr Fillon on his "wide victory".
During the primary campaign, he expressed similar ideas as his rival on the French economy, but tried to rally conservatives around a more tolerant attitude towards France's ethnic, religious and social diversity.
Based on results from about 88% of polling stations, organisers of the centre-right Republicans party primary said Mr Fillon led with 67% of the vote compared to 33% for Mr Juppe.
Mr Fillon's toughest challenge ahead is likely to be far-right leader Marine Le Pen. She is running an anti-establishment campaign that particularly targets immigrants, France's large Muslim minority, and the European Union.
Socialist president Francois Hollande is expected to announce in the coming weeks whether he will seek re-election, but the French left has been deeply weakened by Mr Hollande's extreme unpopularity.
Both Mr Fillon and Mr Juppe are high-profile leaders of the Republicans party who knocked their former boss - Mr Sarkozy - out of the primary's first round of voting a week ago. Mr Sarkozy then threw his weight behind Mr Fillon.
Sunday's run-off came after a bruising and highly adversarial end phase to the months-long primary contest, an American-style effort to end party infighting and bolster support for the party's nominee.
The conservatives previously chose their candidate internally.
Mr Fillon, a conservative Catholic who opposed France's law legalising same-sex marriages, said he plans to reduce immigration to France "to a minimum" - positioning himself firmly to Mr Juppe's right.
Mr Fillon wants to drop sanctions against Russia over its aggressive actions in Ukraine and partner with Russia in the fight against Islamic State extremists. Mr Fillon insists "Russia poses no threat" to the West, while Mr Juppe wants France to continue putting pressure on Mr Putin on various fronts.
They both pledged to cut public spending, reduce the number of civil servants, raise the retirement age from 62 to 65, end the 35-hour work week and cut business taxes.
All French citizens over 18 - whether they are members of the Republicans party or not - were eligible to vote in the primary, if they paid two euro in fees and signed a pledge stating they "share the republican values of the right and the centre".