BUDAPEST, March 31 (Reuters) - Hungarian Prime Minister Viktor Orban's ruling Fidesz party maintained a slim lead over an opposition alliance before Sunday's parliamentary election, a survey by the think tank IDEA Institute showed on Thursday.

The poll, conducted between March 22 and 28, put support for Orban's nationalist Fidesz at 41% of the electorate, up from 40% in early March, while the six-party opposition alliance stood at 39%, gaining two points from the previous survey.

The ranks of the undecided fell by five points to 6%, the poll showed.

Orban faces a united opposition for the first time since he came to power in a 2010 election landslide. Its candidate for prime minister is Peter Marki-Zay, an independent who is now mayor of Hodmezovasarhely, a town in southern Hungary.

Among decided voters, Fidesz held a more comfortable lead, leading the opposition alliance by 5 percentage points. Other polls published earlier this week also put Orban's ruling party ahead of their rivals.

(Reporting by Gergely SzakacsEditing by Tomasz Janowski)