Final election results will now be announced on 8 July - a month later than planned, the National Election Board Ethiopia says.
This will allow election authorities to investigate hundreds of alleged episodes of fraud and vote rigging.
Opposition parties have vowed to stage mass protests if the provisional results are validated.
The opposition have, however, won almost 200 seats - a huge gain from the 12 they had in the previous parliament.
EU observers have voiced concern over irregular vote counting and biased reports by the state-owned media.