CALCUTTA, INDIA. - Hidden in tent camps along East Pakistan's western border with India are the official armed forces of the rebel Bangla Desh government in exile, the Mukti Fouj.

Some regular Indian army units, ignoring the Indo- Pakistan treaty in force since 1947 under which both sides keep their forces five miles from the border, have moved right in beside them.

These are easy to find because their signallers have not had time to bury the field telephone cables that lead from divisional to battalion headquarters and you can follow them from tree to ditch right up to the border.

Parts of the border are particularly ill-defined. In some places it is marked only by a conical-shaped cement block about three feet high standing incongruously in the middle of a paddy field. It is easy to understand the ease with which Pakistani troops hot in pursuit of retreating guerrillas could find, to their cost, that they had invaded India.

With a guide from the Bangla Desh mission in Calcutta (formerly the Pakistan High-Commission, until the Pakistani high commissioner declared himself for Bangla Desh), I went a few miles into East Pakistan with a platoon from a Mukti Fouji base near Bangoan, where some of the biggest refugee camps are.

The Mukti Fouj, which means freedom fighter, came into being when the insurgent East's fight against Pakistan President Yahya Khan's soldiers was about a month old and the East Pakistani Awami League thought it improper that their troops should still be refereed to as the East Pakistan Rifles and the East Bengal Regiment.

At the base I visited, the Mukti Fouj were entirely made up of a company of East Pakistan rifles who had simply walked over the border with most of their equipment. Tents were provided by the Indian government. Pay from the Bangla Desh government is just under $12 a month. Their tents were pitched around some trees about half a mile on the Indian side of the border. Inside, the earth had been packed down to form a floor.

Drainage ditches had been dug around the outside of the tents because the monsoon, which was mercifully late this year, has started and there are heavy showers every day. The men all seemed fit and well fed.

Their commandant was a former captain in the Pakistan Army service corps, a serious looking man in thick, horn-rimmed glasses and tall for a Bengali, who graduated from Kakui Military Academy, in West Pakistan. He asked that his name not be used because, like most of his men, he still had relatives "inside."

The group, which was probably about 100 strong, though the captain would not divulge any figures, claim they control 144 square miles of East Pakistan territory from the India border to the Kapatakha River, which forms a natural boundary between them and the nearest West Pakistan troops.

Most of their activities, said the captain, involved disrupting communications and laying mines. Occasionally they might ambush a Pakistan patrol.

He pointed to two jeeps parked in the camp, which he said had been captured during ambushes. But more often, he said, they just patrolled around "liberated Bangla Desh," ready to go to the assistance of any villagers who were being "harassed by the Paks."