By George Georgiopoulos and David Stamp ATHENS (Reuters) - Greek opposition politicians embarked on the laborious and apparently futile exercise on Saturday of trying to form a new government, oblivious to pressure for rapid elections so the nation can start tackling an array of crises. The two biggest opposition parties, one of the centre-right and the other of the far left, queued up to exercise their constitutional right to spend three days negotiating a new coalition, even though their efforts are almost certain to fail. When leftist Prime Minister Alexis Tsipras resigned on Thursday, he had in mind a quick election next month when he hoped to return to power, strengthened by a mandate to implement the new bailout programme he negotiated to save Greece from financial collapse.