Argentina's President Mauricio Macri was set to make significant gains in mid-term elections Sunday, according to exit polls, giving his center-right coalition a strengthened hand to carry through pro-market economic reforms.

Exit polls by TV channels TN and America 24 showed Macri's Cambiemos, or "Let's Change", coalition making nationwide gains, though neither channel gave figures shortly after polls closed.

"People were able to express themselves on a day of democracy without hardly any problems," said Macri's cabinet chief Marcos Pena.

Full results were expected early Monday.

The election commission put the official turnout at 78 percent for the vote widely seen as a referendum on Macri after two years in office.

Opinion polls had suggested that Macri's Cambiemos, or "Let's Change," coalition could boost their representation in both congress and the senate enough to deprive the opposition of a two-thirds majority to block his reforms, though still fall well short of an overall majority. 

Macri's ruling coalition was set to have the advantage against a center-left opposition led by former president Cristina Kirchner, who is running for a senate seat in the province of Buenos Aires, home to 40 percent of the country's 33.1 million voters.

Kirchner's campaign pitch has been to promise to "put the breaks" on Macri's cuts in tariffs on agricultural exports, deficit spending and loosening of labor laws.

Macri appealed to voters to "not return to the past," attacking his predecessor's populist policies.

"I voted for the government candidates. I'm far from being convinced, but I voted Macri in 2015. The most important thing is that Kirchner doesn't come back," said teacher Sergio Peroti, 48, after voting at a school in the capital.

Rosendo Fraga, a political consultant with the firm Nueva Mayoria, said a victory for Macri "would be a sign that populism is out and that the economic direction is being maintained." 

The vote was overshadowed by the discovery last week of the body of indigenous rights activist Santiago Maldonado, who went missing nearly three months ago after police broke up a protest over tribal land claims.

The case has caused a political storm and revived dark memories of the forced disappearances of opponents during the country's years of dictatorship.

Maldonado's family blame the police for his death and Macri had to face awkward questions as he voted with his wife in Buenos Aires.

"We must let the justice do its work," Macri told reporters after casting his vote. 

"In the past few days, clarity and truth have begun to be shed on what happened and I believe we'll end up knowing everything that has happened in the next few weeks."

- Polarization -

Results were being closely watched in the province of Buenos Aires, an area the size of Italy.

"There is an intensification of the polarization in Buenos Aires," said political analyst Facundo Nejamkis, of the consulting firm Opina Argentina.

Kirchner won the most votes in the province in August 13 primaries, but Macri's coalition was the biggest vote-getter nationally.

Several Argentinian polling firms predicted Macri's forces winning in Buenos Aires by a margin of 3.5 percent.

- Macrinomics -

Up for grabs are half the 254-seat Chamber of Deputies and a third of the 72-seat Senate.

Macri's Cambiemos is an alliance of parties of the right, center-right and social democrats of the historic Radical Civic Union.

Despite not having a legislative majority, Macri has been able to get laws passed by striking deals with Kirchner's enemies in the Peronist movement and with governors who depend on federal funds to finance their budgets.

Macri's first year in office was marked by a 30 percent devaluation of the national currency and a 2.3 percent contraction of the economy.

But the economy has begun to recover, posting 1.6 percent growth in the first half of the year.

However, growth remains below the levels reached before 2010 under the back-to-back husband-and-wife governments of Nestor and Cristina Kirchner, in power from 2003 to 2015.

Macri has gained support for his economic reforms from the United States, the European Union and international lending institutions. He was showered with praise at a recent forum of Argentine business leaders.

- Corruption -

Macri is an engineer by profession from a wealthy family that founded a business empire. 

His launching pad into politics was his chairmanship of the Boca Juniors football club, which won numerous local and international titles under his leadership.

Kirchner, on the other hand, is a former militant in the most combative Peronist factions during the difficult 1970s, when the country was run by a repressive military dictatorship. Her politics moderated with the return of democracy in Argentina.

Kirchner has been prosecuted for alleged corruption in office, but she insists she is the victim of political persecution.

Macri also has faced accusations stemming from the "Panama Papers" scandal, which revealed the rich and powerful's use of offshore accounts to secretly move money and evade taxes. But the complaint against him was quickly dismissed.