BERLIN (AFP) - The German ban on some speculative trading raised economic tensions in Europe on Thursday as unions confronted austerity in Greece with a fourth general strike that had security forces on alert. German Chancellor Angela Merkel took up her campaign for greater regulation on markets and government overspenders at a Berlin meeting with top European officials, having warned this week that the euro is in crisis and banned so-called naked short selling. Merkel, Finance Minister Wolfgang Schaeuble, French Finance Minister Christine Lagarde, who is among those critical of the German ban, and EU economic affairs commissioner Michel Barnier were at the conference. Merkel has caused widespread consternation with her move to ban naked short selling selling bonds or shares which are not owned or are borrowed. The measure concerns notably some German and Austrian government debt bonds. Global financial markets reacted sharply to the German ban, with stocks in the United States, Asia and Europe posting losses and the euro hitting a fresh four-year low of less than 1.22 dollars. Lagarde told RTL radio that France would not follow Germanys unilateral ban and distanced France from Merkels warning that the euro was in danger. Lagarde said the German decision was a measure that should have been taken in concert with other European nations and was in itself open to debate. She said Germany was merely following France in banning the short selling of certain stocks, but that France would not follow Germany in banning it for the sovereign bonds at the heart of Europes current fiscal woes. France feels that markets should remain free to trade in sovereign debt, she said, for reasons of liquidity. Germany fears that short-selling could encourage a run on sovereign debt in debt-ridden states like Greece. Meanwhile, a meeting was being held at the French presidential palace on big deficits in public finances in France. On Wednesday, Merkel had defended her short-selling ban in stark terms. This test is existential and it must be overcome ... if the euro fails, then Europe fails, she told German lawmakers. The euro is in danger. If we do not avert this danger then the consequences are incalculable and the consequences for the whole of Europe are also incalculable. Lagarde insisted though that there is no threat to the euro. I absolutely do not believe that the euro is in danger, Lagarde told RTL radio. The euro is a solid, credible currency that has assured the stability of the eurozone for more than 10 years. Greek authorities meanwhile deployed hundreds of extra police in Athens for the fourth general strike in four months against the governments austerity spending cuts, which caused widespread transport disruption. All major unions have called for rallies in the capital, Athens, and other cities. More than 1,700 extra police were ordered into central Athens alone as authorities seek to avoid a repeat of troubles that marked last strikes. On May 5, a petrol bomb thrown into an Athens bank on the fringes of one rally killed three employees. The Athens metro railway, bus and trolley system all came to a halt, turning the city into a giant traffic jam. In the main port of Piraeus, boats were all tied up in the harbour. International flights were not affected as air traffic controllers decided not to strike, so as not to worsen the impact on the key tourism industry. But Olympic Air cancelled at least 15 internal flights because civil aviation workers joined the stoppage at smaller local airports. Some schools were open and the eduction ministry maintained national examinations for high school students. Some private banks in Athens opened as well despite a call by the main bank workers unioin, OTOE, to join the strike. The main labour federations, the GSEE with one million members, and ADEDY, 370,000 members, called the strike against the shock measures ordered by the government which needed a 110 billion euro rescue from the European Union and International Monetary Fund to avoid a debt default.