"Houthi militants ransacked the party's Sanaa office," the source, from Yemen's Brotherhood-affiliated Islah Party, told The Anadolu Agency.
The Houthis stormed the office on Sunday and took four party members to an undisclosed location.
Since it seized Sanaa last September, the Shiite group has confiscated several of the Islah Party's offices in Sanaa and reportedly attacked other offices in other parts of the country.
Houthi leader Abdel-Malek al-Houthi recently accused the party of colluding with Al-Qaeda inside Yemen.
Tension has mounted in Yemen since the Houthis seized Sanaa last September, from which it has since sought to expand its influence farther afield.
On Feb. 6, the Houthis issued a "constitutional declaration" dissolving parliament and establishing a 551-member transitional council.
The declaration was, however, rejected by most of Yemen's political forces – along with some neighboring Gulf countries – which described it as a "coup against constitutional legitimacy."
Some Gulf States have accused Shiite Iran of backing fractious Yemen's Houthi insurgency.