AMMAN, May 22 (Reuters) - Jordanian security forces have arrested three members of the country's mainstream Islamist party for engaging in acts threatening national security, a senior official said on Tuesday.

The activists from the Islamic Action Front (IAF), which is the country's strongest opposition group, were from the impoverished city of Zarqa, east of the capital, government spokesman Nasser Joudeh told reporters.

"The Islamists were engaged in activities that targeted the security of Jordan and investigations are ongoing," he said.

Zaki Bani Rusheid, head of the IAF, the political arm of the Muslim Brotherhood, said the activists were promoting the party's candidates ahead of next July's municipal elections, in which the Islamists were fielding candidates.

"The authorities want to undermine the Islamic movement and erode any electoral gains," Bani Rusheid told Reuters.

The local polls will also be followed by multi-party parliamentary elections later this year. The Islamists, who have 17 seats in the current parliament, may stand.

The authorities fear the movement, which seeks political change through peaceful means, will make electoral gains in both polls, capitalising on popular disenchantment with successive governments and a belief they failed to deliver on promises of reforms.

Parliament's four-year term formally ends in June, paving the way for the monarch to set a date for elections before the end of the year.

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