FACTBOX: What next in Zimbabwe's election?

(Reuters) - Zimbabwe's June 27 presidential run-off election should be called off because a free and fair vote is impossible, ruling ZANU-PF party defector Simba Makoni said on Tuesday.

Makoni, who challenged President Robert Mugabe in disputed March elections, said opposition MDC leader Morgan Tsvangirai must negotiate a transitional government to rule for five years.

The second round follows a disputed March 29 election in which official results showed Tsvangirai beat Mugabe, but not by enough votes to avoid a run-off.

Below are answers to some questions on the vote:

WILL THE RUN-OFF ELECTION BE CALLED OFF?

Makoni's comment followed a similar call by U.S.-based Human Rights Watch, which said normal campaigning is impossible due to what it called a brutal intimidation and murder campaign by Mugabe's supporters.

The MDC has said intimidation and violence make campaigning increasingly difficult but it will continue to lobby voters ahead of the June 27 run-off.

Despite saying it will contest the run-off vote, the MDC has insisted that it was not necessary because Tsvangirai won convincingly the first time around.

WHAT IS HAPPENING IN THE CAMPAIGN?

Since Tsvangirai returned to Zimbabwe on May 24, he has been detained twice by police while on the campaign trial. Police banned MDC rallies and U.S. and British diplomats were attacked and detained for several hours.

The police ban on rallies has since been overturned by the High Court but the MDC says state-backed violence continues.

Mugabe says he will not allow what he says is a Western-backed opposition party to come to power, dubbing Tsvangirai a British puppet.

HOW FAIR WILL THE VOTE BE?

Tsvangirai's MDC, civic groups, regional bloc SADC and international human rights groups have said post-election violence in Zimbabwe has created conditions making it difficult for a free and fair run-off vote.

The opposition party says over 60 of its members have been killed, while thousands of homes have been destroyed. Mugabe and his ruling ZANU-PF party blame violence on the opposition.

U.S.-based Human Rights Watch said a brutal campaign by Mugabe's supporters had eliminated any chance of a fair vote.