Kenya says an opposition plan to boycott companies linked to allies of President Mwai Kibaki amounts to "illegal sabotage."
In a statement Monday, the government in Nairobi warned opposition leaders they would be held accountable for any property damage.
The opposition Orange Democratic Movement called for fresh demonstrations on Thursday to protest Mr. Kibaki's disputed re-election, but police have banned all rallies.
Also Monday, Kenya's Foreign Ministry summoned the British High Commissioner, Adam Wood, over comments by a member of the British government that London did not recognize Mr. Kibaki's government.
The ministry said Kenya's elections do not need a stamp of approval from the House of Commons.
Ugandan President Yoweri Museveni and former U.N. Secretary-General Kofi Annan plan to visit Kenya this week to try to mediate the political crisis.
Opposition leader Raila Odinga accuses the government of rigging the December 27 election to ensure a Kibaki victory.
Mr. Kibaki has refused opposition demands to resign and his government has banned all political rallies since the start of the new year.
Attempted demonstrations in recent weeks have turned into deadly clashes between police and protesters. In all, more than 650 Kenyans have died in election-related violence.
Much of the bloodshed has been between Kikuyu and Luo tribes. Mr. Kibaki is Kikuyu, while Mr. Odinga is Luo.
Some information for this report provided by Reuters and AFP.