Budapest  - The mayor of Budapest on Sunday issued the capital's first full-scale smog alert since new legislation was passed in December, triggering an automatic ban for almost half of all private vehicles.

As the smog alert was issued on an odd date, only vehicles with odd-numbered registration plates were permitted on the roads of the Hungarian capital on Sunday.

"Prime Minister Ferenc Gyurcsany asks that ministries and state authorities adhere strictly to the smog alert ordinances," said government spokesman David Daroczi.

"Police are stopping traffic at roads into the capital and major junctions in order to warn drivers," said police spokeswoman Eva Tafferner.

Budapest sat under a grey blanket of smog on Sunday after air pollution levels had risen rapidly over several freezing, windless days.

The pollution caused by traffic was exacerbated by power stations on the edge of town that had switched from gas to oil power amid the ongoing gas crisis.

An initial public warning was issued on Saturday as 24-hour average levels of airborne PM10 particulate matter - microscopic dust that forms smog and causes respiratory problems - rose above 75 micrograms per cubic metre for a second day running.

In fact, Hungary's national air pollution monitoring service, which broadcasts real time data from its measuring stations on the internet, was recording 24-hour average PM10 levels of over 300 in some parts of Budapest on Saturday evening.

EU clean air directives set the safe level of PM10 at 50, and by 2010, member states are expected to introduce legislation to prevent air pollution rising above this level on more than 35 days in any given year.

Public transport and essential public service vehicles are exempt from Budapest's traffic restrictions, along with cleaner vehicles such a hybrid cars and those conforming to the EU's stringent Euro 5 emissions standards.

Budapest residents are used to seeing the Hungarian capitals famous landmarks such as the Chain Bridge over the Danube shrouded by a choking grey and yellow mist.

The problem has grown worse over the past decade as car ownership has risen and thousands have moved from the city centre to the suburbs and now commute into Budapest. dpa