By Sudip Kar-Gupta LONDON (Reuters) - European stock markets extended their losing streak on Thursday, with benchmark indexes in Frankfurt and London hitting seven-month lows as fears of a global economic slowdown took hold. The Athens stock market also fell 3.5 percent as the country looked set for new elections on Sept. 20. Greek Prime Minister Alexis Tsipras had been expected to seek early elections to quell a rebellion in his leftist Syriza party and seal support for a painful bailout programme. The pan-European FTSEurofirst 300 equity index closed down 1.9 percent at 1,477.35 points. ...