In the new make-up of Zagreb's 51-strong law-making body, SDP secured 24 seats at local polls held on Sunday.
In order to remain in power, SDP will have to make a coalition with some other councillors. In 2005, it formed a coalition with politician Tatjana Holjevac. After Sunday's local elections, the slate of candidates led by independent Holjevac won four seats, or 6.39 percent of votes, as against 6.97 percent in 2005.
The list proposed by the Croatian Democratic Union (HDZ) and its partners, the Peasant Party (HSS) and the Social Liberals (HSLS), finished second with 13 councillors.
The popularity of the HDZ, which is now led in Zagreb by Jasen Mesic, has risen to 18.43 percent from 15.40 percent in 2005.
The list of the People's Party (HNS), led by its president Radimir Cacic, won 8.18 percent at the last local elections for Zagreb Council as against 8.11 percent in 2005. HNS councillors will hold five seats.
Five seats were also won by the list of independent candidates led by Velimir Srice, an associate of Josip Kregar, who will compete in the Zagreb mayoral runoff on 31 May with SDP candidate Milan Bandic.
In the first round on Sunday, Bandic won 48.54 percent of votes, and Kregar, a law school professor, 23.18 percent.
The HDZ mayoral candidate Mesic followed with 13.17 percent, and Tatjana Holjevac with 5.86 percent.
Other candidates in the Zagreb mayoral race secured less than five percent of votes.