Protesters said US President Donald Trump had "forced" South Korea to deploy the system amid rising tensions with the North.
Choi Soon-sil, the friend of disgraced former South Korean leader Park Geun-hye, was handed the sentence after being convicted of bribery in the first of a string of criminal cases against her.
South Korea's former leader Park Geun-hye faces more than 10 years in jail if found guilty in a corruption trial.
New president Moon Jae-In's government faces a host of challenges including kick-starting the economy, soothing ties with China and dealing with North Korea.
Last polls place left-leaning Moon Jae-in as the front runner. Elections are taking place with a public eager for a fresh start after months of protests to first impeach and then indict Par Geun-hye for corruption.
A snap elections has been called following the impeachment of Park Geun-hye in March over her involvement in one of the country's biggest corruption and influence-peddling scandals.
People who live near the deployment site say the presence of the US anti-missile system is a risk to their security. Candidates running for the May 9 presidential election are split over the issue.
Danish court orders extradition of Chung Yoo-Ra, daughter of Choi Soon-Silm - one of the figures in the major corruption scandal that led to removal of President Park Geun-Hye.
Both Park Geun-hye and Shin Dong-bin, chairman of the retail giant, have denied the accusations that they were involved in exchanges of presidential favours for large sums of money.
Prosecutors now have 20 days to build their case against Park Geun-hye in the corruption scandal that has also led to the detention and trial of Samsung Group chief Jay Y Lee.
Park Geun-hye could face more than 10 years in jail if convicted of receiving bribes from bosses of big conglomerates in return for favours.
The Sewol ferry was structurally unsound, overloaded and travelling too fast on a turn when it capsized and sank during a routine voyage on April 16, 2014.
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