Opponents of a bill giving more powers to Macedonia's ethnic Albanian minority are submitting a petition to force a referendum on the issue.
Opponents say the deal will divide Macedonia along ethnic lines
They say they have collected 30,000 more than the 150,000 signatures required to trigger a vote.
The authorities will now decide whether the petition is valid.
The move poses a serious threat to a 2001 accord that ended fighting between government forces and Albanian fighters, the BBC's Matt Prodger says.
The new legislation, envisaged by the accord, redraws local authority boundaries to increase the representation of ethnic Albanians in Macedonia.
Last month, a large protest against the plan was held in the capital Skopje.
But ethnic Albanians say the changes are essential to improve their position in Macedonia.
In Skopje and other towns across the country, Macedonians queued up to sign a petition demanding a referendum on the deal which has kept a tentative peace for some three years.
Opponents of the deal had until 1400 GMT on Monday to hand in a petition bearing at least 150,000 names - which they said they had easily achieved.
The opposition includes the World Macedonian Congress (SMC), which says it protects "the Macedonian identity".
Some 20,000 opponents of the plan took to the streets in July
"By the end of Saturday we had 178,000 signatures for the referendum," SMC head Todor Petrov told Reuters news agency hours before the deadline was due to expire.
It will take some time for the authorities to verify the petition and a referendum would not be held for at least several weeks.
The multi-ethnic government has said the decentralisation measures in the bill, which will see 16 municipalities pass into Albanian control, are essential for peace and democracy.
But many Macedonians fear the law will make it easier for Albanian minority areas to secede from Skopje and unite with neighbouring Kosovo, should it ever gain independence from Belgrade.
They say it will divide Macedonia along ethnic lines.
The European Union has made it clear that full implementation of the peace accord is essential for Macedonia's membership bid.