ANKARA - Turkish Prime Minister Tayyip Erdogan, facing the biggest protests of his tenure, called on his supporters to prepare for pro-government rallies next weekend in Istanbul and Ankara, as riot police fired tear gas and water cannon to clear protesters from an Ankara square."On Saturday, are you ready for a big Ankara meeting? ... The next day we will have the Istanbul meeting," Erdogan said to cheering crowds in Ankara, as riot police moved in against protesters just a few kilometers away.