How were those chains placed on Baha'u'llah?
An abortive attempt on the life of Násiri'd-Dín Sháh by some crazed Babis in August 1852 led to a general round up of all the known followers of the Bab.
Here is what Abdul-Baha wrote:
Now after the occurrence of this grave matter all of this sect were suspected. At first there was neither investigation nor inquiry, but afterwards in mere justice it was decided that there should be investigation, inquiry, and examination. All who were known to be of this sect fell under suspicion.
Bahá'u'lláh was passing the summer in the village of Afchih situated one stage from Tihran. When this news was spread abroad and punishment began, everyone who was able hid himself in some retreat or fled the country. Amongst these Mirza Yahya, the brother of Bahá'u'lláh, concealed himself, and, a bewildered fugitive, in the guise of a dervish, with kashkul in hand, wandered in mountains and plains on the road to Rasht.
But Bahá'u'lláh rode forth with perfect composure and calmness from Afchih, and came to Niyavaran, which was the abode of the Royal Train and the station of the imperial camp.
Immediately on His arrival He was placed under arrest, and a whole regiment guarded Him closely. After several days of interrogation they sent Him in chains and fetters from Shimiran to the jail of Tihran. And this harshness and punishment was due to the immoderate importunity of Haji 'Ali Khan, the Hajibu'd-Dawlih, nor did there seem any hope of deliverance, until His Majesty the King, moved by his own kindly spirit, commanded circumspection, and ordered this occurrence to be investigated and examined particularly and generally by means of the ministers of the imperial court.
Now when Bahá'u'lláh was interrogated on this matter He answered in reply,
"The event itself indicates the truth of the affair and testifies that this is the action of a thoughtless, unreasoning, and ignorant man. For no reasonable person would charge his pistol with shot when embarking on so grave an enterprise. At least he would so arrange and plan it that the deed should be orderly and systematic. From the very nature of the event it is clear and evident as the sun that it is not the act of such as Myself."
(Abdu'l-Baha, A Traveller's Narrative, p. 30)
Baha'u'llah also described the event as follows in the "Epistle to the Son of the Wolf":
"By the righteousness of God! We were in no wise connected with that evil
deed, and Our innocence was indisputably established by the tribunals.
Nevertheless, they apprehended Us, and from Niyavaran, which was then the
residence of His Majesty, conducted Us, on foot and in chains, with bared
head and bare feet, to the dungeon of Tihran. A brutal man, accompanying
Us on horseback, snatched off Our hat, whilst We were being hurried along
along by a troop of executioners and officials. We were consigned for
four months to a place foul beyond comparison."
~ H.M. Balyuzi, Baha'u'llah - The King of Glory, p. 77