The Southern African Development Community (SADC), which brokered a power-sharing deal between President Robert Mugabe and his rival Prime Minister Morgan Tsvangirai after Zimbabwe's 2008 election descended into violence, urged political players, security forces and the election commission to "exercise restraint and avoid frustrating voters".
"Voters must have unfettered access. That's what will make the elections free and fair. The conduct must go hand in hand with discipline of the highest order," Bernard Membe, who is heading the SADC's election observer mission, told a press conference.
The African Union will also send observers, but Zimbabwe has refused missions from Western nations.
Local election monitors meanwhile called on the Zimbabwe Electoral Commission (ZEC) to extend early voting for thousands of police officers, which started on Sunday but was marred by chaos.
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Polling stations opened late and many lacked indelible ink, stamps, voter rolls and ballot papers and boxes.
The problems were an ominous signal ahead of the upcoming vote, he added.
The group was "seriously concerned that the chaos that prevailed during the special voting process serves as a telling and worrying indicator that could repeat itself," he said.
Police spokeswoman Charity Charamba conceded two days were not enough for the voting.
"Indications on the ground are that it is now clear that members and officers of the Zimbabwe Republic Police have been unable to exercise their constitutional right to vote," she said.