A bomb wrecked a provincial office of Prime Minister Wilfried Martens today, apparently part of a widening series of unusual attacks on business and political targets in Belgium.

Martens was at his home several blocks away when the 2 a.m. explosion tore through the first and second floor of the headquarters of his Flemish Christian Democrat Party in Ghent, 30 miles northwest of Brussels. The bomb reportedly was left at the entrance.

No one was injured in the blast, which was claimed by the Communist Combatant Cells, the same mysterious group that claimed responsibility for four previous bomb explosions this month. The attacks by what appears to be a Belgian group have shocked officials here because domestic dissidents have rarely used such violent tactics.

Martens, in a regular television address this evening, said his government was "strongly concerned about the recent attacks."

"Belgium has always been a model of democracy," he said. "All tendencies have been allowed freedom of expression. This tradition should not be put in danger."

Belgian political parties across the spectrum condemned the attacks as a threat to democratic liberties and called on Martens' center-right government to stand firm against what the French-speaking liberal party called an "odious form of blackmail and intimidation."

The Communist Combative Cells said it attacked its first three targets -- subsidiaries of two American companies and a West German truck manufacturer -- because they were supplying NATO with parts or transport for its medium-range nuclear missile program.

But the group said the attacks Monday against a Brussels office of the liberal party and against the Ghent office were part of the "second phase" of its "October anti-imperialist campaign."

"This is political combat against the bourgeoisie, the forces of capital and the bourgeois dictatorship of the Gol-Martens government," said a letter that a Belgian journalist found after being directed to a Brussels site by a caller.

"Gol" is a reference to Justice Minister Jean Gol, whose liberal party is the Christian Democrats' partner in the coalition government.