Praise be to Allaah.
We appreciate your keenness to convey the message of Islam to
people, and your keenness to implement the Sunnah and not go against
sharee‘ah. We ask Allah, may He be exalted, to make things easy for you and
to grant you the best of rewards.
Praying in congregation in the mosque is obligatory for
everyone who can hear the call to prayer in an ordinary voice without any
barriers and without amplification through loudspeakers. The more correct
scholarly view is that it is only obligatory in a mosque in which the call
for prayer is given.
For more information see the answer to question no.
But there may be some cases in which shar‘i needs or
important interests make it permissible for a group of Muslims to offer
prayer in congregation somewhere other than in the mosque.
There is no doubt that calling non-Muslims to Islam is an
important aim. If you think it most likely that your praying in the lecture
hall will have an impact on them, and may be the cause of some of them
becoming Muslim, then there is nothing wrong with praying in the lecture
hall. So you can give the adhaan and iqaamah, and pray in congregation.
If that is done every year, it is not bid‘ah (an innovation),
because the aim behind it is a legitimate shar‘i aim, namely openly
practising the rituals of Islam and calling non-Muslims to the faith. But
you should not set a specific date for that every year or every month and so
on; rather that should be done according to need. Then you can do it on
different days and choose the best day to get the largest congregation and
so that the largest possible number of non-Muslims can watch.
And Allah knows best.