The Social Democrats scored 22.13 per cent, ahead of the surprise populist newcomer ANO movement of billionaire Andrej Babis with 18.61 per cent and the Communists with 17.08 per cent vying for second place.
The right-wing TOP 09 and ODS were also set to surpass the five per cent threshold needed to enter parliament with 9.4 and 6.73-per cent support, respectively.
Dawn of Direct Democracy, a populist party led by Japanese-born businessman Tomio Okamura, scored 7.53 per cent. Christian Democrats scored 6.79 per cent.
Voters already swung left in January, electing former Communist Milos Zeman as president after a decade under the right-wing and eurosceptic Vaclav Klaus.
Despite an apparent Social Democrat victory, it was unclear today whether they would team up with the Communists or ANO.