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By MacDonald Dzirutwe

HARARE, May 6 (Reuters) - An independent Zimbabwean election monitoring group expressed doubt on Tuesday over the credibility of the results of the presidential election and accused the ruling ZANU-PF party of attacking observers.

The Zimbabwe Election Support Network (ZESN) is the first observer group to publicly question the results, which showed opposition leader Morgan Tsvangirai had failed to beat President Robert Mugabe by enough to avoid a second round run-off.

The observer group, made up of 38 non-governmental organisations, said a one-month delay in announcing the results of the March 29 poll undermined the impartiality of the Zimbabwe Electoral Commission (ZEC).

"ZESN cannot substantiate ZEC figures as the network is not aware of the chain of custody of the ballot materials during the aforementioned period," ZESN said in a statement.

"In addition, the delay to announce the results was a major concern, not only to ZESN but the general public as well and this obviously undermined the impartiality, credibility and transparency of ZEC."

The official results, announced on Friday, gave Movement for Democratic Change leader Tsvangirai 47.9 percent of the vote to Mugabe's 43.2 percent.

The MDC says Tsvangirai won the election outright and has ended Mugabe's 28-year rule over the once prosperous country whose economy is now in ruins.

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"ZESN is also aware that there was no transparency in the verification, collation and tabulation of presidential results, as party agents who are required by the law to be present were not invited to witness the process," said the group.

The MDC has yet to say whether Tsvangirai will contest a second round against Mugabe, but has said one condition for doing so might be a U.N.-led observer mission.

To pull out now would mean Mugabe automatically maintained his hold on power.

Zimbabweans who had high hopes the election would usher in a new era are now victims of a protracted political battle between their wily president and Tsvangirai, who has proved indecisive.

Tsvangirai has been touring African countries seeking support from leaders to help push out Mugabe, not demonstrating leadership at home where severe food and fuel shortages and hyper-inflation have broken millions of Zimbabweans.

Mugabe faces growing pressure from Western powers.

U.N. Secretary-General Ban Ki-moon said on Monday he was talking to African states about how the world body could help ensure an election run-off in Zimbabwe was credible and voiced concern at growing violence.

The regional SADC group of nations, who have the best chance of influencing Mugabe, might be ready to press him to accept U.N. monitors. But the 84-year-old, who critics accuse of using security crackdowns to keep a grip on power, has not budged.

The MDC has accused ZANU-PF of a violent campaign to intimidate Zimbabweans into voting for Mugabe in a runoff. The ruling party alleges MDC supporters have engaged in political violence.

ZESN called on police to stop attacks on election observers.

"The attacks are happening now and so far 8 huts have been torched with property being destroyed. Running battles between observers and these youths are currently in progress," it said.

"Youths from ZANU-PF have been on the rampage targeting and physically beating up anyone thought to have observed the March 29 election." (Reporting by MacDonald Dzirutwe; Writing by Michael Georgy; Editing by Robert Woodward)

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