KARACHI, Pakistan, April 15 —The Pakistani radio said tonight that Pakistani troops had prevented “Indian infiltrators and antistate elements” from dynamiting the strategic Bhairab railroad bridge in East Pakistan.

In unusually detailed broadcast, the radio said that “Indian interventionists” were now “aiming mainly at the dislocation of the transport and communications system in East Pakistan” to hurt the economy and hamper the army.

But the radio said that elsewhere Pakistani troops had pushed northward to the remote town of Thakurgaon near the borders of India and Nepal and had cleared out “troublemakers in the area.”

And in general, it said, “all principal towns” in the East were “under the effective control of the Pakistani Army.” “Remaining strongholds of the antistate elements are being dealt with suitably,” it added.

The report of the drive to Thakurgaon, coming after yesterday's account of a sweep. to the Ganges, also showed that the army was seeking to quell as rapidly as possible hostile elements near East Pakistan's border with India.

Yet, today's broadcasts indicated that at least some hostility remains. The Government gave this account of events at the Bhairab bridge, which spans the Meghna River and links Dacca with Chittagong, East Pakistan's main port:

“In predawn lightning action today, the Pakistani armed forces secured the strategic IBhairab bridge, after inflicting heavy casualties on the.Indian infiltrators and antistate eleIments poised to dynamite it. Hugh quantities of their arms, ammunition, equipment and rap tions were also captured.

“The infiltrators and antistate elements had placed 38 dynamite charges at various points of the bridge and just as they were about to blow it up troops surprised them and took the bridge intact. They captured demolition charges bearing the markings of Indian ordnance factories, which were obviously prepared by experts of the Indian Army. Pakistan troops after securing the ends of the bridge have advanced further to mop up all remnants of the infiltrators in the area.”

To the north, the radio said, the 30 miles of frontier area from Dinajpur to Thakurgaon were “cleared of all groups of troublemakers.”

Two days have passed since Pakistan announced that her army had “destroyed all miscreants” in the area of Dinajpur, a larger town on the rail line south of Thakurgaon. Twenty miles east of the Indian border, Thakurgaon is also only 35 miles southeast of the mountain kingdom of Nepal.

Stressing that events in East Pakistan were Pakistan's private affair, the radio said, “an international Red Cross team wanted to take relief supplies into East Pakistan and the Pakistan Government refused it permission to do so.”