The election will give Ukraine's voters the first chance to pass judgement on Western-oriented President Petro Poroshenko's attempts to pull the former Soviet republic out of its worst crisis since independence in 1991 and away from historical master Moscow.
"The mission will assess the entire election process for compliance with OSCE commitments and other international standards for democratic elections, as well as with national legislation," the Organisation for Security and Cooperation in Europe said.
"Observers will monitor the legislative framework and its implementation, the work of the election administration and relevant government bodies, campaign activities, media coverage and the resolution of election disputes," the Vienna-based body said.
Led by Tana de Zulueta of Italy, the team consists of 16 experts based in Kiev and 80 long-term observers deployed across Ukraine. Another 600 short-term observers will monitor election day proceedings.
The vote will give the 450-seat parliament additional powers such as nominating prime ministers without prior consultation with the president, making its cooperation essential for the former chocolate tycoon.
A new People's Front created by Prime Minister Arseniy Yatsenyuk and the president's eponymous own party are expected to form the bedrock of the current leadership's parliamentary support.
But the pro-Russian Regions Party that ruled Ukraine under former president Viktor Yanukovych -- whose ouster in February sparked the current crisis -- has said it will boycott the vote.
The OSCE has emerged as a key player in Ukraine, sending observers to the restive east of the country and taking part in peace talks in Minsk two weeks ago that resulted in a so-far shaky truce.
The pan-European security body, whose 57 members include Russia, Ukraine and the United States, aims to double the number of observers in the east to 500 in the next 10 weeks and also plans to deploy drones.