The story of Abu Dharr Ghifaris (radiallahu ‘anhu) conversion to Islam as told by Abdullah Ibn Abbas (radiallahu ‘anhu) shows how perilous it had become even to call upon the Prophet (peace be upon him) in those days.
Thereupon he took some provisions, together with an old waterskin full of water, and proceeded to Mecca. Then he went to the mosque (Kaba) and began exploring for the Prophet (peace be upon him) for he knew him not and was reluctant to ask about him and thus he spent part of the night. Thereupon Ali saw him and recognized the same to be a stranger and when Abu Dharr met Ali, he went with him (to the latters house). Until daybreak, neither of the two asked any questions of each other. Then once again he (Abu Dharr) went with his waterskin and his provisions to the mosque and allowed that day to pass til evening without finding the Prophet, although the latter saw him from there. Then he returned to his resting-place. While in there, Ali passed by him and said: Is it not time that a man should know his abode? And his remarks made him rise and finally brought him to his house, with neither of the two asking any questions of each other, too. And on the third day Ali did likewise, and he (Abu Dharr) stayed with him. Thereafter (Ali) said: Will you not tell me what had brought you here? to which Abu Dharr answered: I will do so only if you promise me that you will guide me right, whereupon Ali agreed outright. After that, Abu Dharr told him all. Then, Ali said: Behold, it is true, and he is indeed an Prophet (peace be upon him) of God! Tomorrow morning, you follow me. If indeed I see any danger for you, I will stop as if to pass water; but if I go on, then follow me and enter in whichever place I do. Abu Dharr did so, following Ali until he finally found the Prophets house and entered in it together with him (Ali). Then he listened to some of the Prophets sayings and embraced Islam the spot. Thereupon the Prophet (peace be upon him) said unto him: Return unto thy people and inform them about me and await my bidding. Afterwhich Abu Dharr said: By Him in whose hand is my soul, indeed I shall loudly proclaim the truth among them!
Then he left and went to the mosque and called out at the top of his voice. He proceeded to say: I bear witness that there is no deity but God, and that Muhammad is His Prophet. Thereupon, the people of Mecca broke upon him, beat him and then threw him into the ground. At such juncture, Al-Abbas came, knelt down to see him and said to the people: Woe unto you, know ye not that he belongeth to the tribe of Ghifar and that your merchants road to Syria passeth through their country? Thus, Al-Abbas rescued him from them. That incident did not stop Abu Dharr from doing the same thing again, prompting the people (of Mecca) to impose themselves over him anew, whereupon Al-Abbas came once more to his redemption. (Al-Bukhaari, Section: Abu Dharrs conversion to Islam).