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Zambian police have fired tear gas and made 28 arrests during a raid on the home of opposition vice-presidential candidate Geoffrey Bwalya Mwamba.

 Officers were looking for people they allege had vandalised election posters and fled to the property.

A police spokesman said petrol bombs were also found, but Mr Mwamba's party has said they were planted.

There have been growing concerns over political violence in Zambia ahead of next month's general election.

The raid comes just two days after a ban on political campaigning in parts of the country had been lifted, reports the BBC from the capital, Lusaka.

Mr Mwamba, who was not in the house in Kasama in the country's Northern Province at the time, said his grandchildren were injured during the raid.

"My small grandchildren have been taken to hospital as they have been victims of this barbaric act carried out by the police," he said in a post on his Facebook page.

Mr Mwamba is the running mate of Hakainde Hichilema, the presidential candidate of the United Party for National Development (UPND).

Northern Province Police Commissioner Bonnie Kapeso said the police officers had to use tear gas in order to get the people they were chasing out of the house.

He added that five petrol bombs were found as well as spears and machetes.

The UPND said in a statement that "the items allegedly found were only recovered once the house was cleared. We know these have been planted". Zambians go to the polls on August 11 to elect a president and a new parliament.

Incumbent President Edgar Lungu, from the Patriotic Front, is facing a tough challenge from Mr Hichilema.