The public fast service is structured around bringing the Ark out, placing ashes on it and the leader, and delivering words of supplication and repentance. This is an early rabbinic witness to institutionalized communal supplication, distinct from the daily Tachanun of the later siddur.
סֵדֶר תַּעֲנִיּוֹת כֵּיצַד, מוֹצִיאִין אֶת הַתֵּבָה לִרְחוֹבָהּ שֶׁל עִיר, וְנוֹתְנִין אֵפֶר מִקְלֶה עַל גַּבֵּי הַתֵּבָה, וּבְרֹאשׁ הַנָּשִׂיא וּבְרֹאשׁ אַב בֵּית דִּין, וְכָל אֶחָד וְאֶחָד נוֹתֵן בְּרֹאשׁוֹ. הַזָּקֵן שֶׁבָּהֶן אוֹמֵר לִפְנֵיהֶן דִּבְרֵי כִבּוּשִׁין, אַחֵינוּ, לֹא נֶאֱמַר בְּאַנְשֵׁי נִינְוֵה, וַיַּרְא הָאֱלֹהִים אֶת שַׂקָּם וְאֶת תַּעֲנִיתָם, אֶלָּא (יונה ג) וַיַּרְא הָאֱלֹהִים אֶת מַעֲשֵׂיהֶם, כִּי שָׁבוּ מִדַּרְכָּם הָרָעָה. וּבַקַּבָּלָה הוּא אוֹמֵר (יואל ב) וְקִרְעוּ לְבַבְכֶם וְאַל בִּגְדֵיכֶם: עָמְדוּ בִתְפִלָּה, מוֹרִידִין לִפְנֵי הַתֵּבָה זָקֵן וְרָגִיל, וְיֶשׁ לוֹ בָנִים, וּבֵיתוֹ רֵיקָם, כְּדֵי שֶׁיְּהֵא לִבּוֹ שָׁלֵם בַּתְּפִלָּה, וְאוֹמֵר לִפְנֵיהֶם עֶשְׂרִים וְאַרְבַּע בְּרָכוֹת, שְׁמֹנֶה עֶשְׂרֵה שֶׁבְּכָל יוֹם, וּמוֹסִיף עֲלֵיהֶן עוֹד שֵׁשׁ:
What is the customary order of fast days? Normally the sacred ark in the synagogue, which was mobile, was kept in a locked room. However, on fast days they remove the ark to the main city square and place burnt ashes upon the ark, as a sign of mourning. And they also place ashes on the head of the Nasi, and on the head of the deputy Nasi, and each and every member of the community likewise places ashes upon his head. The eldest member of the community says to the congregation statements of reproof, for example: Our brothers, it is not stated with regard to the people of Nineveh: And God saw their sackcloth and their fasting. Rather, the verse says: “And God saw their deeds, that they had turned from their evil way” (Jonah 3:10). And in the Prophets it says: “And rend your hearts and not your garments, and return to the Lord your God” (Joel 2:13). This teaches that prayer and fasting are insufficient, as one must also repent and amend his ways in practice. They stood for prayer. The congregation appoints an elder, who is experienced in leading prayer, to descend before the ark as communal prayer leader. And this prayer leader must have children and must have an empty house, i.e., he must be poor, so that his heart will be fully concentrated on the prayer for the needs of his community. And he recites twenty-four blessings before the congregation: The eighteen blessings of the everyday Amida prayer, to which he adds another six blessings,