Terrorists planted 40 time bombs in three Mexican cities, and 23 exploded in the pre-dawn hours today, causing extensive damage, police said. Many of the targets were government offices and American-owned businesses.

At least five people were injured, including two policemen and a suspected terrorist.

The bombings, in addition to five kidnapings and one killing, came on the eve of Mexico's independence celebrations, a date marked in past years by outbreaks of violence.They appeared to accelerate a trend of political violence in recent years.

Police reported that kidnapers seized two doctors and three other persons in towns around Mexico City late yesterday, and the wife of one doctor was killed by the abductors.

Police said six bombs exploded in Mexico City. The targets included the Federal Justice Tribunal and the Confederation of Chambers of Commerce. Unexploded bombs were found outside a General Motors assembly plant and the headquarters of the Colgate Palmolive subsidiary.

Special correspondent Marlise Simons reported that while insurgent groups in the countryside have been a normal feature of Mexico's modern history, urban guerilla warfare began about six years ago. Since then Mexico's major cities have had a wave of bank robberies, kidnapings for high ransoms, bombings and killings of policemen.

Frequently the urban terrorist groups have been broken up by police and clearly identified as extreme left. But much so-called leftist guerilla action has also turned out to be the work of nonpolitical criminals while the left has often suggested that it was rightists who had more to gain from creating social chaos and from the repression that followed. Today, for example, Mexico's Communist Party charged that the bombs had come from "groups linked to the CIA who are interested in disturbing the peace on the eve of Mexico's independence day celebration."

The bombings came at a politically sensitive moment as the government is launching a substantial political reform program, that will legalize the Communist Party and other leftist opposition groups. Key politicians in the ruling Institutional Revolutionary Party are strongly opposed to opening up the political arena they have dominated for more than 50 years. The far left also opposes the reforms.

Police said a hitherto-unknown group calling itself the Union of the People distributed leaflets taking responsibility for the bombings in Mexico City, Guadalajara and Oaxaca.

Earlier, authorities thought the attacks were a joint effort by three guerrilla groups, including the 23rd of September Communist League and the Armed Revolutionary Forces of the People. The Union of the People pamphlets said, however, that the bombings were the first concentrated action by that group and called other guerrilla bands "cowards of the left."

The pamphlets said attacks would continue to protest unemployment and hunger.

The 23rd of September League, which takes its name from the date of a gun battle with soldiers a dozen years ago, has become the largest of urban guerrilla organization. Two years ago intelligence estimates placed its membership at about 50, but police have arrested more than that number without slowing the group's activities.

The heaviest damage was at a police station in sururban Netzahualcoyotl, where two policemen were injured.

Ten bombs went off in Guadalajara, the site of much urban terrorism in recent years.

One bomb set off a fire that raged for several hours through Guadalajara's largest department store, causing $2 million in damage. Other targets included a Sears Roebuck store, a Woolworth store, the Mexican-American cultural institute, government buildings and banks.

The bomb at the Woolworth store exploded in the hands of a suspected terrorist, who was badly injured and placed under arrest.