Should one stand during Kerias Hatorah?
The congregation is not required to stand during the Torah reading [and so is the widespread custom, and so is the ruling according to Kabbala]. However, there are some who are stringent to stand [during the actual Torah reading, although even they are allowed to sit between the Aliyos]. [Practically, the Sefaradi custom, and custom of many Ashkenazim and Chassidim is not to stand during the reading, although some Ashkenazim and Chassidim are stringent to stand. This applies also according to the Chabad custom, that one may sit during the reading, although some are stringent to stand. Regarding the Chabad custom in a Shul that does not have a Bima platform-see next.]
No Bima platform: The above ruling [that one is not required to stand] applies even if there is no elevated platform in the Shul, known as the Bima, and the Torah is simply read on a table, [or reading table figuratively called a Bima] which is of equal floor level as the rest of the congregation. However, some Poskim, rule that if the Sefer Torah rests on a table and not on a Bima, then one is required to stand from the letter of the law, and may only sit between the Aliyos. However, those who are weak and cannot concentrate properly on the reading, may sit even in such a case.
The congregation is not required to stand during the reading, although some are stringent to stand. Some Poskim learn that if the Sefer Torah rests on a table and not on a Bima, then one is to stand during the reading, and may only sit between the Aliyos. However, those who are weak and cannot concentrate properly on the reading, may sit.
May one be stringent to stand if the entire congregations sits?
Some Poskim rule that if the entire congregation sits during the reading one may not be stringent to stand. Other Poskim, however, rule that one may stand even in such a case, and so is the custom of those who are stringent.
May one who is lenient to sit do so even if the entire congregation stands?
Some Poskim rule it is permitted to do so.
May one sit opposite the Olah or Baal Korei?
One is not to sit or even stand opposite the Olah.
 Shulchan Aruch 146:4
 Beis Yosef 141:1; Tur 146:4; Peri Chadash 146:4; Mateh Yosef 146:9; Kaf Hachaim 146:20 that so is custom today amongst all Jewry, including Chassidim and Anshei Maaseh
 Shaar Hakavanos Derush 1 p. 48 that the Arizal was accustomed to sit throughout the entire Kerias Hatorah, unlike those accustomed to stand; Peri Eitz Chaim 14:1; Mishnas Chassidim Yom Sheiyni 5:8
According to Kabalah must one sit down or is it optional? Some Poskim understand that accoridng to the custom of the Arizal, one is specifically to remain seated during the reading, and it not simply that he is not required to stand. [Mishnas Chassidim ibid “They are not to stand”; Pesach Hadvir 146:3; Chesed Lealafim 135:14; Ruach Chaim; Shulchan Hatahor 146:4; Kaf Hachaim 146:22] Others, however, negate this understanding and rule that even accoridng to Kabala one may stand if he chooses. [Sdei Chemed Mareches Beis Peas Hasadeh 29]
 Michaber 146:4; Beis Yosef 146:4 and 141:1 that so is implication of Rashi Megillah 21a and that so is the custom; Tur 146:4 in name of Sar Shalom [as explained in Bach ibid]; Hamanhig Hilchos Shabbos 30 p. 158; Mordechai in Halachos Ketanos 968; Gloss of Rabbeinu Peretz on Tashbeitz 182; Implication of Yerushalmi Megillah 4:1; Perisha 141:1; Biur Hagr”a 146:4; Peri Chadash 146:4; Levush 146:3; Mateh Yosef 146:9; Pesach Hadvir 146:3; Chesed Lealafim 135:14; Ruach Chaim [Falagi]; Shulchan Hatahor 146:4; Shaareiy Efraim 4:9
 Stringency or letter of law: Some Poskim learn that even according to those who are stringent, it is not due to that they view this matter as an obligation from the letter of the law, but rather simply as a proper act. [Bach 141:1] However, other Poskim seem to learn that it is required from the letter of the law. [See Taz 146:1; Lechem Chamudos Brachos 36; Rameh 91, brought in Elya Raba 146:4; Kaf Hachaim 146:20;]
 M”A 146:7; Bach 141:1 that between the Aliyos everyone agrees that one may sit; Peri Chadash 146:4; Kneses Hagedola 146; P”M 146 A”A 7; Kaf Hachaim 146:20
 Rama 146:4 and that so was the custom of the Maharam of Rothenburg; Teshuvos Upesakim of Maharam 503; Tashbeitz Katan 182 that so was custom of Maharam; Mordechai Shabbos 422 in name of Maharam and that so is custom of some German and French Jews; Besamim Rosh 290 in name of Maharam; Custom of some recorded [and negated] in Tur ibid in name of Sar Shalom; Taz 146:1 that the opinion of Maharam is correct and so is proper to be followed; Teshuvas Rav Ovadia Seforni 90, brought in Taz ibid, that one must stand; Peri Chadash 146:4 concludes that one is allowed to be stringent like Maharam; Seder Hayom Kerias Hatorah Shabbos, brought in Kneses Hagedola 146, that he saw some meticulous people standing, and it is a good and proper custom; Lechem Chamudos Brachos 36; Rameh 91 that so is proper custom, brought in Elya Raba 146:4; Drech Chaim 76:1
The reason: As the verse in Nechemia 8:5 states “Ubepischo Amdu Kol Ha’am” which they interpret to mean that when the Sefer Torah is open one is to stand. However, in truth, the Talmud [Sotah 39a] itself states that the intent here of the word “stand” is to be silent and not speak, and hence the above reason is negated. [Hamanhig ibid; Mordechai in Halachos Ketanos ibid; Tur ibid; Beis Yosef ibid; Gr”a ibid] However, the Taz ibid defends this reason, saying the intent of the Talmud is to say that the standing must be done quietly, and it is not coming to exclude standing. Alternatively, the reason is because when we received the Torah on Sinai we stood [Devarim 5:5] and the Zohar states that when the Torah is read it is similar to receiving it on Sinai. Thus, when one hears the Torah reading he must imagine as if he is standing by Sinai and this is what led some to stand during the reading, as it is fit and proper to stand just like we stood on Sinai. [Bach 141:1]
 The Chabad custom: When the Rebbe was addressed the question of whether one is to stand for Kerias Hatorah, he motioned the asker to look at the ruling of the Rebbe Rashab in the Siddur. [Igros Kodesh 24:120; Shulchan Menachem 1:261] There, the Rebbe Rashab records the custom of the Arizal to sit, although then mentions the need to be stringent if there isn’t an elevated Bima platform. In Hayom Yom [24th Shvat; 1st Shavuos; 13th Menachem Av] it says to stand by Shiras Hayam and Aseres Hadibros, implying that by other times one may sit. The custom of the Chabad Rabbeim: The Rebbe Rashab was almost always accustomed to stand during Kerias Hatorah. [Reply of Rebbe Rayatz to Rebbe, printed in Reshimos Hayoman p. 184] However, on Rosh Hashana and Yom Kippur, he was accustomed to sit during the Keria. [Reshimos Devarim 1:102] The Rebbe Rayatz even in Rostov was witnessed to sit during Kerias Hatorah of Shachris and stand for the reading of Mincha. [Rav S.Z. Gurary, recorded in Chikrei Minhagim p. 176] The Rebbe until 1978 was accustomed to stand during the entire reading, including between the Aliyos. However, from Simchas Torah of 1978 and onwards [after the heart attack] the Rebbe began to sit, and did so for all years thereafter. [See Yalkut Kerias Hatorah Lefi Minhag Chabad [Printed in Chazak Chumash] p. 1374; Chikrei Haminhagim 1:176-186]
 M”A 146:6 that even on the Bima itself only the reader must stand; Beis Yosef in name of Rashba; Tosafus Menachos 33; Taz Y.D. 282, brought in Machatzis Hashekel ibid, that a Bima which is ten Tefach high is considered another Reshus; Kaf Hachaim 146:24
 Siddur Rav Shabsi; Rebbe Rashab in Siddur Torah Or p. 488 printed in Siddur Im Dach p. 640; The Rebbe in Igros Kodesh 24:120 referenced the asker to look in the gloss of the Rebbe Rashab in response to whether one must stand by Kerias Hatorah.
 Rebbe Rashab in Siddur Torah Or p. 488 printed in Siddur Im Dach p. 640
 Kol Eliyahu 5; Ikarei Hadaat 6:15; Pischeiy Teshuvah 146:2; Orchos Chaim 146:2
 Pesach Hadvir 140:3; Kaf Hachaim 146:22; Imrei Bina 13:4; Dvar Yehoshua 2:15; Betzel Hachochma 5:1; Az Nidbaru 6:43; Piskeiy Teshuvah 146:6
 Betzel Hachochma 5:1; Piskeiy Teshuvah 146:6
 Rebbe Rashab in Siddur Torah Or p. 488 printed in Siddur Im Dach p. 640