At a White House event at noon, Mr. Obama announced plans to introduce legislation by next week that includes a ban on assault weapons, limits on high-capacity magazines, expanded background checks for gun purchases and new gun trafficking laws to crack down on the spread of weapons across the country.

He also promised to act without Congressional approval to increase the enforcement of existing gun laws and improve the flow of information among federal agencies to keep guns out of the hands of criminals and others who shouldn’t have them.

The announcement was the culmination of a monthlong process that began after the massacre of 20 children at an elementary school in Newtown, Conn. In the wake of the shootings, Mr. Obama pledged action, but it was not immediately clear how far he was willing to go in the face of intense political opposition.