Thousands of protesters have descended on Malaysia's capital – despite the closing of key roads – to attend a banned rally demanding electoral reforms.

Police in Kuala Lumpur on Saturday fired rounds of tear gas and used water cannons to disperse the crowds. Toxins in the chemically-laced water used in the cannons hang in the mist, making an uncomfortable environment for demonstrators.

Police say more than 500 people have been detained.

Leaders of Bersih, a coalition of more than 60 non-governmental groups, say they will go ahead with the rally, even though the government has refused to issue the necessary permit.

Meanwhile, pro-government supporters are conducting their own demonstration in the capital without police intervention.

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.