The leader of the junior partner in Japan's ruling coalition expressed concern Saturday over the stock rallies and rapid weakening of the yen following the Bank of Japan's decision Friday to ease monetary policy even further.

"Side effects are bound to accompany" the developments in the stock and foreign exchange markets when such changes happen rapidly, said Komeito party leader Natsuo Yamaguchi during a party meeting held in Mito, Ibaraki Prefecture.

Yamaguchi suggested the need to see out the effects of the announced BOJ move on the economy, saying the easing measures must lead to increased business investment and to wage increases.

Also Saturday, the main opposition Democratic Party of Japan criticized the central bank's surprise decision to provide more liquidity to the economy, with party leader Banri Kaieda warning the move will accelerate "Japan selling" in financial markets.

During a party meeting in Utsunomiya, Tochigi Prefecture, Kaieda characterized the BOJ decision as a "prohibited move" because of its impact on the livelihoods of Japanese people.

"It was a very risky decision," the opposition leader told reporters after the meeting. "The BOJ should not do things that would hurt the value of the yen."

The BOJ said Friday that it will expand its asset purchase program to counter downward pressure on prices as it seeks to raise the inflation rate to 2 percent by sometime next year.