Passing in Front of Someone in Prayer
It is forbidden to pass within four amot (about 6 feet) of a person praying the Shemoneh Esrei. This rule applies specifically to passing in front of the person. However, it is permitted to pass or stand on the side of the worshiper. (Shulchan Aruch 102:4)
The Mishnah Berurah mentions two reasons for the above ruling: 1) Passing in front of one who is praying disturbs the concentration of the worshiper. According to this reason one is not permitted to pass in front even if there is a glass partition separating him from the worshiper since the he will still be disturbed (Kaf HaChaim in the name of several halachic authorities). 2) One who passing causes a separation between the Divine Presence (Shechina) and the worshiper. According to this reason, even if his face is completely covered by his tallit, and he will not be disturbed, it is still forbidden to pass in front of him. (Kaf HaChaim)
Even though the Shulchan Aruch writes that is permitted to pass or stand on the side of one praying the Shemoneh Esrei, the Mishnah Berurah writes that according to the Zohar it is forbidden. (The Mishnah Berurah writes at the end of the first halacha that it is possibly forbidden to stand within four amot of someone praying since it disturbs his concentration. Yet, from what he writes above (in halacha 4) in the name of the Zohar, it would seem that it is certainly forbidden to stand within four amot in front of someone praying. This is so since it is unlikely that the rule of standing in front can be more lenient than standing to the side.)
The Kaf HaChaim adds that according to the Zohar it is forbidden to pass in front of one praying as far as the eye can see, except in a pressing situation. It is interesting to note that both the Magen Avraham and Shulchan Aruch HaRav, like the Mishnah Berurah, mention the ruling of the Zohar forbidding passing on the sides, but do not mention passing in front as far as the eye can see.
The halachic authorities write that just as it is forbidden to pass in front of someone praying, it is also forbidden to pass in front of one reciting the verse “Shema Yisrael” (Eliya Rabbah, Mishnah Berurah, Kaf HaChaim). The Kaf Hachaim writes in the name of Teshuvot HaGeonim and other poskim that it is forbidden to pass in front of the shaliach tzibur who leads the prayers, even during parts of prayer other than the Shemoneh Esrei.