The Zimbabwe prime minister's party says a new opinion poll reflecting a surge in support for President Robert Mugabe's party was skewed by a "margin of terror" that saw nearly half those questioned refusing to disclose their political loyalties.

Douglas Mwonzora, spokesman for Prime Minister Morgan Tsvangirai's party, said Thursday that U.S.-based Freedom House poll is "unreliable" in a climate where voters face intimidation by Mugabe militants and loyalists dominating the police and military.

In a 54-page report, Freedom House said support for Mugabe's ZANU-PF party rose to 31 percent from 17 percent in the past 18 months but dropped for Tsvangirai's party from 38 percent to 20 percent.

It said 1,198 Zimbabweans were polled with a plus or minus 2.8 percent margin of error.