ATHENS - Agents for Greek betting firm OPAP, Europe's biggest, went on strike on Thursday over a tax on winnings and said a new smoking ban in public places in Greece had added to a fall in sales.

There was already a drop due to the global downturn but with the smoking ban the fall came to 25-30 percent (year on year), OPAP sales agents union head Kyriakos Toptsidis told Reuters.

The union, which represents about 5,300 retail outlets selling lottery cards and sports betting games across Greece, went on strike to protest a tax on betting winnings.

Greece passed a law this week which scrapped tax-free betting winnings and set the tax at 10 percent, a move which could hit OPAP's profit.

The smoking ban took effect on July 1.

Toptsidis said all OPAP retail outlets were closed and called on the government to repeal the tax.

The participation rate is close to 100 percent, he said, adding that agents had also seized OPAP's data processing centers in Athens and Thessaloniki, the country's two biggest cities.

He said the new tax would discourage players from wagering their profits in OPAP's flagship games Stihima and Kino.

Analysts are concerned OPAP could be forced to raise the payout on Stihima to boost the game, which already faces stiff competition from online bookmakers.

A source from the Greek betting monopoly said its board was expected to meet later in the day over the strike.

Toptsidis said the strike could mean daily losses of about 5 million euros in sales for OPAP.

 OPAP shares were down 1.53 percent at 16.10 euros as of 1246 GMT, off a low of 16.04 that marked their lowest level since November 2004.

They trade at 7 times estimated 2009 earnings, compared with 22.6 times for Lottomatica and 5 times with Ladbrokes , according to Reuters Estimates.

(Editing by Jason Neely)