The number of protesters was higher than many had expected

Amid driving rain and tight security, protesters demanded the resignation of government lawmakers debating the plan.

The parliament is considering redrawing municipal borders to give minority Albanians more power in certain areas.

The protests pose a threat to a 2001 peace deal between Albanian rebels and the army to stave off civil conflict.

The bill being reviewed by parliament is a key part of that accord, which was brokered by western states as civil war threatened in the former Yugoslav republic.

But many Macedonians fear the law will make it easier for Albanian minority areas to cede from Skopje and unite with neighbouring Kosovo, should it ever gain independence from Belgrade.

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.

The BBC's Matt Prodger in Skopje says the government has little room for manoeuvre, as its ambition of moving closer to the European Union rest upon the implementation of the 2001 peace deal.

Struga strife

Thousands marched in Skopje on Monday after parliament adjourned its debate for the day.

Police have warned against any repeat of the Struga violence

Many carried Macedonian flags and listened to speeches accusing the government of bowing to the demands of ethnic Albanians.

Security was tight to prevent violence of the kind that erupted last week in the city of Struga, one of the municipalities where control could pass into Albanian hands.

Violent protests in Struga left dozens of protesters and police officers with minor injuries and forced Defence Minister Vlado Buckovski to be evacuated from the scene.

'Earthquake'

Macedonian opponents of the decentralisation plan say it will divide the country along ethnic lines and give too much power to Albanians.

As well as losing control over two towns, Macedonians would see the capital become a bilingual city after its boundaries are expanded to bring in more Albanian villages.

Macedonian newspapers have described the bill being negotiated between the government and Albanian politicians as a "national earthquake" - a reference to the natural disaster that killed thousands in Skopje on 26 July 1963.

Our correspondent says the issue goes to the heart of the dispute which almost led Macedonia to civil war: how to balance the rights of the country's Albanian minority against the interests of the Macedonian majority.