* Conservative Pinera wins 36.6 pct, center-left Guillier 22.7 pct
* The two will face off in Dec. 17 runoff
* Guillier seeks to rally fractured left to defeat favorite Pinera (Adds context, fresh comments from Pinera, Guillier and other candidates)
By Felipe Iturrieta and Dave Sherwood
SANTIAGO, Nov 19 (Reuters) - Conservative billionaire Sebastian Pinera will face center-left Senator Alejandro Guillier in a runoff for Chile's presidency next month, after a first round vote on Sunday that Pinera won, but with fewer votes than expected.
Both candidates would keep in place the top copper exporter's longstanding free-market economic model, but former president Pinera has promised investor-friendly policies to turbocharge growth, while Guillier wants to press on with outgoing President Michelle Bachelet's overhaul of education, taxes and labor.
With over 98 percent of votes counted, Pinera, a 67-year-old businessman, had clinched 36.6 percent, falling short of the 50 percent needed for an outright victory, Chile's electoral agency Servel said.
Guillier, a 64-year-old bearded former TV news anchor elected to the Senate in 2013, had 22.7 percent, just over two points ahead of the third placed candidate, leftist Beatriz Sanchez, whose better than expected showing will likely ensure her Frente Amplio coalition will yield sway in Congress.
Pinera's result was below pollster expectations, indicating that the Dec. 17 runoff will likely be a more closely-fought contest than previously forecast, especially if Guillier can rally supporters of Sanchez and four other left-leaning rivals behind him.
"We're going to have a very competitive second round," Pinera's campaign chief Andres Chadwick told journalists on Sunday evening.
The most recent opinion survey by CEP last month had forecast Pinera securing 42 percent of likely votes in the first round, and easily defeating Guillier in the runoff.
After a divisive race, Guillier called for "profound unity" to unite Chile's fractured left against Pinera.
"There are more of us, and therefore we must win in December!," Guillier told his cheering supporters.
Sanchez did not endorse Guillier in a fiery speech before her followers, even as she pilloried his opponent. "Sebastian Pinera is a step backward for the country and the country is going in a different direction!" Sanchez said.
But Pinera, who previously led Chile between 2010 and 2014, flashed the optimism he has shown on the campaign trail, where he has repeatedly plugged his campaign slogan "better times."
"This result is very similar to the one in 2009. And you'll remember that in 2009 we won the election," Pinera said, before a buoyant crowd.
Voter turnout was higher than expected, a factor which analysts forecast will count against Pinera if repeated in a runoff.
Bachelet cannot run again this year because of term limits. Guillier was one of two candidates her center-left coalition Nueva Mayoria backed in the first round.
Speaking from the presidential palace on Sunday evening, she urged Chileans to vote in December, saying her policy changes were at stake.
Guillier faces a more polished opponent in Pinera, who oversaw robust economic growth in his first presidential term that overlapped with higher copper prices.
Pinera has vowed to double Chile's economic growth rate, with proposals that include trimming corporate taxes, making state-run miner Codelco more self-sufficient, and tweaking the pension system to include incentives for later retirement.
But he must ease fears that he would wipe out gains made in Bachelet's government for students, women and workers - from expanding free education to strengthening unions.
Should Guillier triumph in the runoff, he has pledged to tackle stubbornly high inequality in one of Latin America's most business-friendly economies. He wants to diversify Chile's dependence on copper, increase access to free higher education and write workers' rights into a new constitution.
"I voted for Guillier because I think we have to continue to provide free education. It's a social right," said unemployed voter Mario Giannetti, 53.
Since its transition to democracy from dictatorship in 1990, Chile has stood out as one of the region's most developed countries. But public debt has grown as lower copper prices hit government revenues in recent years, and Bachelet's critics say she failed to court investments or prioritize economic growth that slowed to an annual average of 1.8 percent.
In a first since the 1990s, credit ratings agencies Fitch and S&P downgraded Chile's debt rating this year.
While investors see Pinera as a safe pair hands for the economy, his previous term was marred by massive student protests seeking an education overhaul. His responses were often seen as out of touch and grassroots groups still oppose him.
Early on Sunday, a group of young protesters stormed Pinera's campaign headquarters before being arrested by police, TV images showed.
Reporting by Dave Sherwood, Felipe Iturrieta and Antonio de la Jara, writing by Mitra Taj; editing by Rosalba O'Brien