Halakha Yesaya 1:28: Mishneh Torah, Prayer and the Priestly Blessing, Kitzur Shulchan Arukh & Shulchan Arukh, Orach Chayim

Every procedure of a sacred character should only take place in a congregation of Israelites, as it is said; "And I will be sanctified in the midst of the children of Israel" (Leviticus 22:32). All the recitals, referred to above, if begun when a quorum of ten was present, of whom some left, though they are not authorized to do so, are concluded by those who remain.

As soon as he has begun to read in the Torah, it is forbidden to converse even on a Halachic topic. All must listen silently and give heed to what he is reading, as it is said, "And the ears of all the people were attentive unto the book of the Law" (Nehemiah 8:3). It is likewise forbidden to leave the synagogue while the Torah is being read. But one may leave in the interval between one section and another. A person who is continuously engaged in the study of the Torah, this being his sole occupation, may continue his study while the Law is being read.

If there is only an exact minyan, in the synagogue it is prohibited for any of them to leave. Of one who leaves it is said:²⁴Isaiah 1:28. "Those who abandon God will be consumed." But, if ten will remain, the rest may leave if they have already heard Barechu, Kedushah and the Kaddishim after Aleinu. In the event that less than ten were left they may conclude the prayers, even with less than ten (provided that they are a majority of a minyan). If the chazzan had started to repeat the Shemoneh Esrei, he may complete the entire repetition of the Shemoneh Esrei and the congregation may say Kedushah, and he (the chazzan) also says: Elokeinu veilokei avoseinu barecheinu baberachah etc.²⁵These are the Kohanim blessings. However, the kohanim do not bless the congregation.²⁶Lit. “Raise up their hands”—the position of the Kohanim while blessing the congregation. Also, the Kaddish that follows the Shemoneh Esrei is not said²⁷According to Mishnah Berurah, both the half-Kaddish and the whole Kaddish are said. because these are different subjects. If they began the Torah reading with ten men and some [congregants] leave, the reading may be completed, but no additional [aliyahs] are permitted, and no one is called for maftir, but, rather, the last of the allotted olim reads the Haftarah without the berachos.