A suicide bomber has killed at least 85 people and injured about 150 in an attack on a campaign rally in south-western Pakistan, officials say.

They say a provincial candidate was among the dead in the town of Mastung.

Earlier, a bomb attack on another election-related gathering killed four people in the northern town of Bannu. There are fears of further violence ahead of general elections on 25 July.

Meanwhile, former PM Nawaz Sharif was arrested after flying home from the UK.

Sharif and his daughter Maryam were taken into custody by officials from the National Accountability Bureau (NAB) after landing in the northern city of Lahore.

The three-term PM was ousted last year after a corruption investigation. Last week he was sentenced in absentia to 10 years in prison.

He has accused Pakistan's powerful security establishment of conspiring against him ahead of the elections.

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What is known about Friday's attack?

The attack in Mastung was the deadliest such incident in Pakistan in more than a year. There are fears that the death toll will rise further.

Among those killed was Baluchistan provincial assembly candidate Siraj Raisani, his family said.

He was a candidate for Balochistan Awami party.

Earlier in the day, a campaign convoy of another candidate was attacked in Bannu.

Akram Khan Durrani, who represents the MMA party, was unhurt, officials say.

Unexpected attacks

Analysis by BBC's M Ilyas Khan in Islamabad

Coming amid claims by the army that militants have been cleared from Pakistani areas, these attacks are unexpected.

Before the 2013 elections, militants had issued early warnings, resulting in a muted campaign by targeted parties.

Those same parties and groups appear to be targets again. These attacks may also trigger a new set of tensions ahead of an election already marred by controversy.

Significantly, all attacks have come in the aftermath of the conviction of former Prime Minister Nawaz Sharif by an anti-corruption court.

Friday's attacks came just hours before Sharif flew in from the UK to Lahore, and the city is under a lockdown ordered by authorities to prevent his supporters from staging a huge welcome for him.

On Tuesday, a suicide bomber hit a campaign gathering in the northern city of Peshawar, killing 22 people - including a provincial candidate.

The Pakistani Taliban said it had carried out that attack.

Tensions across Pakistan have been rising ahead of Sharif's return to the country.

Thousands of police officers have been deployed and shipping containers have been placed on some main roads in to block supporters from reaching the airport.

On Thursday, Sharif's PML-N party said hundreds of activists in Lahore had been detained ahead of Sharif's return.

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• A war of nerves between Pakistan's military and Sharif

Pakistan's general election

• Voters will elect candidates for the 342-seat Pakistan National Assembly
• The main parties are Nawaz Sharif's PML-N, former cricketer Imran Khan's PTI and Bilawal Bhutto Zardari's PPP
• It will mark the second time that one civilian government has handed power to another after serving a full term
• The run-up to the vote has been marred by what observers say is a crackdown on political activists, journalists and critics of the powerful military
• More than 371,000 troops will be deployed to protect the election and ensure it is "free and fair", the army says

BBC.COM