PRAGUE (Reuters) - Czech President Milos Zeman told supporters on Thursday he would seek a second term in 2018, ending speculation over whether he would run again.

Zeman, who has pushed for better ties with Russia and backed Donald Trump in the U.S. vote last year, announced his decision at an annual party at Prague Castle to celebrate the anniversary of his election in 2013, two participants who asked not to be named told Reuters.

Other media outlets also reported Zeman's comments.

"I announce to you that I decided to run again for the office of president," Zeman told the gathering, according to mobile phone video shown on Czech Television.

Zeman will hold a news conference on Friday to officially announce his decision to the public.

Zeman, 72, won his country's first ever direct presidential election four years ago, remains its most popular politician and is in strong position for the January 2018 election.

While governments run domestic and foreign policy, the president can hold leverage over the European Union country's cabinets, as well as appoint central bankers and nominate ambassadors and constitutional court judges.

Zeman's first term has been marked by battles with a Social Democrat-led government, sniping at journalists, warnings about Muslim immigration to Europe and efforts to improve relations with Russia and China.

He opposes EU sanctions imposed on Russia over its annexation of Crimea in 2014 and its role in a rebellion in eastern Ukraine - measures which the center-left cabinet backs. He was one of the few European leaders to vocally back Trump's campaign last year.

ELECTION SEASON

Zeman will play a role in naming the next prime minister after a parliamentary election in October in which no party is likely to secure a majority. Some analysts say Zeman may tie this to support for him.

A former Social Democrat leader, Zeman split with the party after it failed to completely back him in his first attempt at the presidency in 2003 in a vote among parliamentarians. He ran as an independent four years ago.

Zeman so far has only one serious challenger next year: businessman and writer Michal Horacek, who launched his campaign last November with the motto "We can do better".

Zeman was out of politics for a decade before his 2013 victory. Instrumental in his comeback were several businessmen with interests in Russia, including the president's top adviser Martin Nejedly, who did business in Moscow and had a joint venture with Russian oil firm Lukoil in the Czech Republic.

Zeman was the only EU leader to attend a World War II victory ceremony in Moscow in 2015.

He has also helped steer the country toward China and welcomed Chinese President Xi Jinping to Prague last year.

This week, the richest Czech businessman, Petr Kellner, met with Zeman to discuss economic ties with those countries, both of which Zeman aims to visit this year. In April Zeman will meet Trump at the White House.

(Reporting by Robert Muller and Jason Hovet; editing by Andrew Roche)