MEXICO CITY (AP) -- A strong earthquake shook Mexico City today, sending thousands of people fleeing into the street as buildings swayed. The National Seismological Service tentatively established the magnitude of the 4:41 p.m. EDT quake at 6.7.  There was no immediate report of major damage or of injuries in the capital. Telephone service was disrupted in Mexico City and local television reported that power was knocked out to much of Puebla, a large city about 60 miles east of Mexico City.