The officials said the rebels, also known as Houthis, attacked the Islamist Islah Party headquarters in Ibb province and fought with the guards until early Saturday.
Mediators managed to stop the fighting and allow those who were inside the party headquarters to leave, but the Houthis later detained 25 people and took them to the office of the provincial governor, a Houthi himself.
The Houthis blew up a nearby dispensary belonging to the Islah Party, forcing hundreds of people to flee the area, the officials said on condition of anonymity because they are not allowed to brief the media.
Ten of Yemen's Western and international allies urged the Houthis Saturday to stop their campaign to seize government and military institutions and allow the government to exercise its duties without threats or intervention in its work.
In a statement, the group said that the Yemeni government is the only power responsible for its national security and called for the speedy formation of a regime in line with an agreement signed by all political parties last September.
The statement, signed by "The Group of Ten Ambassadors," referred to countries that backed a power-transfer deal that saw former President Ali Abdullah Saleh relinquish power after a yearlong uprising in 2011.
Meanwhile, hundreds of Yemeni men and women demonstrated Saturday in the capital Sanaa, demanding Houthis to withdraw their fighters from Sanaa and other provinces and to allow the government to run the country's affairs peacefully.
The Houthis waged a six-year insurgency in the north against Saleh, which officially ended in 2010, but after his ouster, they fought ultraconservative Islamists in several northern cities and towns. AP