Can the khateeb beinterrupted in his Khutbah on Friday?
Question

The sanctity of the hour of Jumu-ah, can this be disrupted at any stage? If so when and for what reasons?
Praise be to Allah.Firstly: speaking during the khutbah when the khateeb is speaking on the minbar is haraam and the one who does that is committing a sin, even if he speaks words of Dhikr (remembrance of Allah) at this time.
 Jumu’ah should be a time of sitting still and keeping quiet, and the worshipper should be humble in body and soul and listen to the exhortation and knowledge which the general Muslim public needs to hear. It is not permissible to speak even if it is to enjoin what is good and forbid what is evil. Even though this (enjoining what is good and forbidding what is evil) is one of the most important duties of the Muslim, it is not allowed at this time (during the khutbah) and one cannot even say “Shush!” or “Listen!” The evidence (daleel) for that is as follows: 
It was reported from Abu Hurayrah that the Prophet (peace and blessings of Allah be upon him) said: “If you say to your companion ‘Listen!’ whilst the imaam is preaching on Friday, you have spoken laghw (idle talk).” (Narrated by al-Bukhaari, 892; Muslim, 851).
 Look – may Allah bless you – even if you say to a person, “Listen!” – which is enjoining what is good and forbidding what is evil – Islam counts this as laghw (idle talk) which is forbidden at the time of Jumu’ah.
 The matter is even more serious than that. Listen to the hadeeth which comes next:
 It was reported that Abu’l-Dardaa’ said: “The Prophet (peace and blessings of Allah be upon him) sat on the minbar and addressed the people, and recited an aayah. Ubayy ibn Ka’b was beside me and I said to him, ‘O Ubayy, when was this aayah revealed?’ He refused to answer me. I asked him again and he ignored me, and I asked him again and he still ignored me, refusing to speak to me until the Messenger of Allah (peace and blessings of Allah be upon him) had come down from the minbar. Then Ubayy said to me, ‘All you got from your Jumu’ah was speaking idle talk (laghw).’ When the Messenger of Allah (peace and blessings of Allah be upon him) left, I went to him and told him what had happened. He said, ‘Ubayy spoke the truth. If you hear your imaam speaking, then listen to him until he finishes.’” (Narrated by Ibn Maajah, 1111; Ahmad, 20780. Classed as saheeh by al-Busayri and by Shaykh al-Albaani in Tamaam al-Minnah, p. 338). 
Asking about an aayah on the day of Jumu’ah cancels out the reward of Jumu’ah, so how about people who speak about their business or crops or other worldly matters? Another kind of negligence on some people’s part is when they make the khutbah a time for taking a nap and cannot enjoy their sleep except during the time of Jumu’ah.
 Even saying “Yarhamuk Allah” (may Allah have mercy on you) to someone who sneezes, or returning salaams during the time of Jumu’ah is not permitted. Al-Nawawi says in al-Majmoo’: 
The saheeh texts forbid saying “Yarhamuk Allah” to someone who sneezes and returning salaams. (Tamaam al-Minnah, p. 335).
 The same applies to all adhkaar (dhikr) such as istighfaar (seeking forgiveness) or tasbeeh (saying Subhaan Allah) etc. It is not permissible to say them out loud even though this is dhikr. The khutbah is a kind of dhikr or remembrance of Allah, because Allah says (interpretation of the meaning):
“O you who believe (Muslims)! When the call is proclaimed for the Salaat (prayer) on the day of Friday (Jumu’ah prayers), come to the remembrance of Allah…” [al-Jumu’ah 62:9]
This includes the khutbah and the prayer; both of them are remembrance of Allah (dhikr). Other kinds of dhikr such as tasbeeh etc., are Sunnah and may be done at all other times; the khutbah and listening to it, on the other hand, form an obligatory (waajib) form of dhikr which is prescribed for a short and specific time, during which focusing on the khutbah takes precedence over focusing on any other kind of good deed or worship.
 Similarly, saying “Ameen” to the imaam’s du’aa’ and sending blessings on the Prophet (peace and blessings of Allah be upon him) if he is mentioned during the khutbah, should be done silently by the members of thecongregation, not out loud.
 Secondly: the ban on speaking and making dhikr applies whilst the khateeb is actually speaking on the minbar. If he is on the minbar but he is not speaking, then there is no prohibition on speaking and remembering Allah (making dhikr), because it says in the hadeeth quoted above, “…whilst the imaam is preaching…”
 So the ban on speaking is confined to the time when the imaam is actually giving the khutbah.
 Concerning the hadeeth, “When the khateeb ascends the minbar, there should be no prayer and no speaking” – this is a false hadeeth with no basis. (al-Silsilat al-Da’eefah, p. 87).
 Thirdly: occasions when it is permitted to speak or move whilst the khateeb is speaking from the minbar:
If a person is faced with some need which he cannot put off dealing with, such as drowsiness, the need to relieve himself, or pain which means he has to move. The evidence (daleel) for that is the hadeeth: “If any one of you become drowsy in the mosque on the day of Jumu’ah, let him move from where he is to another spot.” (Narrated by Abu Dawood, 1119; al-Tirmidhi, 526. Classed as saheeh by Shaykh al-Albaani in al-Silsilah al-Saheehah, 468).
Al-Bayhaqi added the phrase “and whilst the imaam is giving the khutbah”. This was also classed as saheeh by al-Albaani.
It is permissible to do things which are permitted even during the prayer, such as guiding a blind person so that he does not fall; or paying attention to the necessities of life in matters which may lead to death or may affect the common good – such as the worshippers asking the imaam to pray for rain for them.
 
It was reported that Anas ibn Maalik said: a man entered the mosque on the day of Jumu’ah from the direction of the courtyard whilst the Messenger of Allah (peace and blessings of Allah be upon him) was standing and giving the khutbah. He turned to the Messenger of Allah (peace and blessings of Allah be upon him) and said, “O Messenger of Allah, our wealth (livestock) has been destroyed and our means of transportation (camels) has been cut off; pray to Allah to help us.” So the Messenger of Allah (peace and blessings of Allah be upon him) raised his hands… (Narrated by al-Bukhaari, 967; Muslim, 897).
It is permissible for a member of the congregation to correct the imaam if he makes a mistake in an aayah, or to prompt him if necessary, whilst he is standing on the minbar. It is also permissible to refute the khateeb if he says something that amounts to shirk, bid’ah (innovation) or munkar (reprehensible error) during the khutbah, so long as that will not lead to a greater evil or fitnah happening in the mosque. If that is the case, then one should wait until after the khutbah and then speak to the imaam, pointing out what is wrong. If the imaam speaks words of falsehood, it is not obligatory to listen to him. It was reported that some of the Salaf used to speak when the oppressor al-Hajjaaj was cursing ‘Ali (may Allah be pleased with him) on the minbar; they said, “We were not commanded to listen to this!”
It is permissible to pray Tahiyyat al-Masjid (two rak’ahs “greeting the mosque”); indeed, this is obligatory, even if the khateeb is on the minbar giving the khutbah.
This is because of the hadeeth of Jaabir ibn ‘Abd-Allah who said: a man came whilst the Prophet (peace and blessings of Allah be upon him) was addressing the people on the day of Jumu’ah. He said, “Have you prayed, O So and so?” He said, “No.” he said, “Stand up and pray two rak’ahs.” (Narrated by al-Bukhaari, 888; Muslim, 875).
If a person sees another person talking, it is not permissible to tell him to be quiet by speaking to him, as stated above, but one can indicate to him to be quiet by making a gesture, such as placing one's finger on one’s lips.
And Allah knows best.