St Petersburg marked earth hour by turning out the lights of Palace Square. Climate change protesters held a candle lit vigil at 8.30pm local time.
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The IMF's forecast for global growth has been cut to 3.1% - a fifth successive year of declining growth, the weakest performance since 2009.
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Fresh violence has erupted in the West Bank following the death of a 13-year-old boy, sparking fears of a new Palestinian uprising.