HARARE —  Early results from Zimbabwe’s constitutional referendum signalled widespread backing for laws to curb President Robert Mugabe’s powers, though the vote count was marred by fresh political arrests on Sunday.A preliminary count of around half a million votes showed more than 90 percent endorsed the new constitution, according to Prime Minister Morgan Tsvangirai’s party.The proposed text would introduce presidential term limits, beef up parliament’s powers and pave the way for a general election to decide whether 89-year-old Mugabe stays in power.Mugabe has ruled uninterrupted since the country’s independence in 1980, despite a series of disputed and violent polls and a severe economic crash propelled by hyper-inflation.Mugabe has backed the proposed constitution which, while curtailing his powers, would also allow him to remain in office for another decade until he is 99 -- elections permitting.His political rival Tsvangirai has also lent his support to the text, but has voiced concerns that a continued crackdown by Mugabe’s security apparatus could derail elections scheduled for July.The latest sign of that intimidation came Sunday as plainclothes police detained four of Tsvangirai’s senior aides.Police confirmed the arrest of Thabani Mpofu, an advisor to Tsvangirai and three subordinates in the premier’s office as well prominent human rights lawyer Beatrice Mtetwa."Today, this morning we arrested four people for impersonating the police,” national police spokeswoman Charity Charamba told journalists. "They are currently in police custody. We have also arrested Beatrice Mtewa for obstructing or defeating the course of justice.”