Police in Malaysia's capital have detained more than 400 protesters and have fired tear gas on crowds of people in an effort to prevent a banned rally demanding electoral reforms.

Police have sealed off key Kuala Lumpur roads and deployed tucks mounted with water cannons near the stadium where electoral reform activists plan to hold Saturday's rally. Demonstration organizers say they expect tens of thousands of protesters.

Authorities have obtained court orders to bar 91 opposition and civic leaders from entering Kuala Lumpur's downtown area.

Leaders of Bersih, a coalition of more than 60 non-governmental groups, say they will go ahead with the rally at the city's iconic Merdeka Stadium, even though the government has refused to issue the necessary permit. The coalition had reached an agreement with Malaysia's constitutional monarch to hold the rally in the stadium instead of the streets, in order to prevent a clash with police.

Bersih is demanding the government implement electoral reforms ahead of the next round of national elections, including measures to prevent vote-buying and fraud. The elections are due to be held in 2013.

More than 200 people have been arrested in the past few weeks for helping to support the rally, drawing criticism from human rights advocacy groups.