(Adds arrests of suspected militants in Lahore)

By Zeeshan Haider

ISLAMABAD, Feb 10 (Reuters) - Pakistani police said on Sunday they suspected Islamist militants based in the tribal areas on the Afghan border were behind a suicide attack that killed up to 20 people at an election rally in the northwest a day earlier.

The lone bomber blew himself up in the midst of a rally of the opposition Awami National Party (ANP) in Charsadda town in the North West Frontier Province (NWFP).

Provincial police chief Sharif Virk told Reuters that the severed head of the bomber had been found, and militants from the nearby Mohmand tribal region could have been responsible.

Pakistan votes for a new National Assembly and four provincial assemblies on Feb. 18, and, while it is not a presidential election, President Pervez Musharraf's future could be at stake if the polls result in a hostile parliament.

ANP is a secular, ethnic Pashtun party competing with Musharraf's allies who were in charge of the central government and religious parties who have held power in NWFP.

A party spokesman said ANP believed the attack was part of a plot to spoil or delay elections that the parties in power were likely to lose.

"This attack is carried out by the forces who want to subvert elections," Zahid Khan, ANP's top spokesman, said.

The attack took place inside a house where ANP leaders were addressing around 200 supporters.

"The bomber appears to be standing in a corner and all of a sudden pushed his way to the middle and blew himself up," Farman Ali, deputy mayor of Charsadda, said.

Pakistan's lawless tribal regions are known as safe havens for al Qaeda and Taliban militants, who fled Afghanistan after U.S.-led strikes following the Sept. 11 attacks and took refuge with ethnic Pashtun tribes across the border.

Security forces have been battling militants in tribal areas since 2003, but after the army crushed a militant movement at Islamabad's Red Mosque last July violence has escalated in towns and cities across Pakistan, particularly in the northwest.

More than 400 people have been killed in militant-related violence since the start of this year alone.

Saturday's suicide attack was the third in the tiny town of Charsadda in less than a year. Two earlier attacks targeted Aftab Ahmed Khan Sheparo, a former interior minister and close ally of President Pervez Musharraf.

Meanwhile, police said on Sunday they had arrested around 10 suspected militants belonging to Lashkar-e-Jhangvi, an outlawed Sunni Muslim militant group linked to al Qaeda, in a raid on their hideout in the eastern city of Lahore.

"We have recovered suicide jackets and other weapons from them. They had planned to attack political and religious gatherings," a senior police official in the city said. (Editing by Alex Richardson)

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