Story highlights Authorities accuse the officials of impersonating police

The arrests come a day after a key vote on a new constitution

Prime minister's office says the officials were arrested after a police raid

Police raided an office of Zimbabwe's prime minister on Sunday, arresting four officials a day after a key vote in the African nation

The officials are accused of impersonating police, Zimbabwe police spokeswoman Charity Charamba told reporters.

The raid came a day after a national referendum to approve a new constitution that would impose presidential term limits for the first time in Zimbabwe. The results of the voting are not yet known. And it came just a few months before Prime Minister Morgan Tsvangirai is expected to run against President Robert Mugabe in presidential elections.

Tsvangirai's office said in a statement that the officials were arrested after a raid on his communications office. Police also arrested Beatrice Mtetwa, a prominent human rights lawyer, accusing her of "defeating the course of justice," Charamba said.

"She went where police were conducting searches and she was shouting at the detectives," Charamba said.

The officials are in police custody as investigations continue, Charamba said.

Mugabe, 89, has been in power for decades, first serving as prime minister in 1980 and taking over as president seven years later.

He and Tsvangirai are expected to face off in the upcoming election, which will end a fragile coalition between them.

The two leaders entered a power-sharing agreement in 2009 after regional leaders nullified Mugabe's victory, citing violence by his loyalists targeted at rival supporters.

Saturday's referendum -- one of the conditions set before an election -- clears the way for the first poll since a disputed outcome plunged the nation into chaos in 2008.

If approved, which is highly likely, the new constitution will give more powers to the parliament and limit the president's. It also introduces a two-term limit of five years each for a president.

But the limit will not apply retroactively, which means Mugabe could have 10 more years as head of state if he is re-elected.