HARARE, Zimbabwe -- Zimbabwe's police and military have mounted an "alarming clampdown" on basic freedoms ahead of crucial national elections planned for the end of July, world rights group Amnesty International said Friday.

Noel Kututwa, an Amnesty director for Africa, said the organization documented clear partisan support for President Robert Mugabe by top security commanders along with "systematic raids and arbitrary arrests" targeting human rights and democracy activists.

Kututwa said politically motivated arrests on flimsy charges served to keep political activists tied up in the courts, paralyze them with legal costs and instill fear of being detained in the nation's notorious jails and police cells. He said such arrests were "explicitly forbidden" under a new constitution overwhelmingly accepted in a referendum in March that demands an end to bias by the security services and guarantees freedom of expression and free political activities.

"Government agencies remain actively hostile to civil society," Kututwa said.

He called on security forces to respect and protect fundamental freedoms.

Mugabe proclaimed the July 31 poll despite calls by regional mediators for a delay to allow the completion of democratic reforms and a clean-up of chaotic voters' lists. Prime Minister Morgan Tsvangirai has been in a shaky coalition with Mugabe since the last violent and disputed polls in 2008.

Tsvangirai's party has called for an extension of the voter registration that closed on Tuesday, leaving tens of thousands of new potential electors off their district voting lists, known as the voters' roll.

An independent research group said Friday preliminary voters' lists for July 31 compared against the results of a national population census last August indicated as many as two million potential voters under the age of 30 were eligible for registration in the past month. Only a small proportion of these were likely to have actually been registered.

"There are well over 1 million people on the roll who are either deceased or have departed" from their districts, said the Harare-based Research and Advocacy Unit.

It said it found voters' rolls in 63 of 210 districts known as parliamentary constituencies had more voters than the inhabitants who were counted in the official population census.

The state election commission said in June it had removed more than 300,000 dead people from the lists used in 2008 that contained 5.7 million names.

In past elections, polling officials have been accused of vote rigging by using flawed voters' lists.

In a routine travel advisory to its nationals in Zimbabwe, the British government on Friday urged them in the run-up to elections "to avoid associating with any activity that could be construed as political."

"Political tensions remain. You should avoid all demonstrations and rallies. In the past, the authorities have used force to suppress demonstrations," it said.

It asked Britons living in Zimbabwe to review their personal security arrangements, make sure travel documents are up to date and "have a contingency plan in case you need to leave at short notice."