Tires set alight by mobs protesting rising food prices burn in a street in Maputo, Mozambique, on Wednesday. (Nastasya Tay/Associated Press)

Police opened fire and killed six people after stone-throwing mobs filled the streets of Mozambique's capital city Wednesday to protest rising food prices.

The marches were illegal and no group had sought permission to hold them, police said.

Thousands of people, mostly men, lined the streets north of downtown Maputo, burning tires and looting shops as they made their way into the city centre.

Police appealed for calm and said they had made an unspecified number of arrests. Youths were blocking streets and public transport drivers abandoned their vehicles in the streets.

International food prices are at their highest in two years, the UN's Food and Agriculture Organization said Wednesday.

In Mozambique, the price of a loaf of bread rose 25 per cent in the past year to about 14 cents. Fuel and water costs also rose.

The FAO's food price index shot up five per cent between July and August, partly because of a drought in Russia that lifted the cost of wheat, it said.

The international food price surge also reflects higher sugar and oilseed prices, the FAO said.

Government blamed for high prices

In Mozambique, critics say bad government decisions have made shortages worse and have accused producers of colluding to push prices up.

The FRELIMO party, in power since Mozambique won independence from Portugal in 1975, has been plagued by charges its government is corrupt and inefficient.

Violent protests over high costs erupted in Maputo in 2008, as well, when global food prices jumped. The government cut fuel prices after a week of clashes police and rioters that left four people dead and more than 100 seriously injured.