The Rabbanut has publicized its call to light the Lag b'Omer fires not on Motzei Shabbos, but on late Sunday afternoon. Locally in Bet Shemesh, the call was publicized by Rav Spektor, the rav of the Givat Sharret neighborhood. In addition, there was a letter signed by many local rabbonim stating that they support the Rabbanut's initiative to push off the bonfires from Motzei Shabbos to Sunday late afternoon and by doing so will increase shmiras shabbos. Note: the list of rabbonim signed on the letter were of shuls associated with the Dati Leumi communities.

Rav Spektor's letter, translated not by me, was publicized on the various Ramat/Bet Shemesh email lists, and can be seen here:

1. The Moetzet haRabbanut haRashit announced that since lag ba'omer comes out this year on a motzash, bonfires should only be lit on Sunday towards evening (but preferably before shekiya)
This decision was made in order to avoid any chance of Hillul Shabbat. The education ministry accepted this decision and gave the children Monday as a day off school to catch up on sleep {ed. note, we do NOT know whySunday was given off also!}  The fire department is also preparing for Sunday evening bonfires, not Motz"ash!
2. It is assur to collect wood on Shabbat. All preparation should be done motzash, or preferably on Sunday-dayIf one sees others making a mistake about this, speak to them KINDLY.
3. The Torah forbids stealing . In order for a bonfire to be Kosher, the wood can only be wood that belongs to the people involved, or taken from true Hefker, and NOT from public places. Workers are NOT at liberty to give wood that belongs to the homeowner for whom they are working, nor from their boss.
4. It is also assur to hurt others, therefore one may not make a bonfire close to homes, which would cause people at home to breathe smoke, which is a health hazard
5. V'Nishmartem Me'od L'nafshoteichim - all people attending a bonfire must be careful to stay far enough away to avoid burns, and to be extra careful with small children. Much credit is due to Ezrat Ahim and Keren Yosef for promoting awareness of fire safety. If there is any fear that a fire might be spreading, one MUST call the firestation immediately (dial 102)
6. Kedoshim t'hiyu - make sure that what goes in and out of the mouths at the bonfire is kosher - the food, the talk and the form of celebration - simhat mitzva.
7. B'agala u'vizman kariv - Just as one may not steal wood, one may not steal (aka use without permission) shopping agalot / carts/ buggies. If one did accidentally used such a buggy, one should make sure to return it AS SOON AS POSSIBLE (bizman kariv) 
8. Lifnei iveir lo titein michshol - make sure to clean up after the bonfire, in order to avoid leaving behind anything that could be a michshol / obstacle / safety hazard to others. Altogether, one must be mehader in keeping the mitzva fo yishuv ha'aretz , according to the passuk "zeh keili v;anveihu", and keep Eretz Yisrael beautiful and clean

I addition, I received by email some similar letters that had been issued by rabbonim of other communities around Israel. The main theme being the need to push off the bonfires to avoid potential chilul shabbos, there was an additional point made in another letter, and a point I heard verbally in the name of other rabbonim. The additional point is that perhaps we need not be concerned about the potential chilul shabbos because we will take great care, in our communities, to ensure that all preparations are done before Shabbos or after Shabbos, and absolutely no chilul Shabbos will occur.

And I am 100% sure that the various frum communities themselves will not engage in any chilul shabbos in the preparations for Lag b'Omer. the problem is not specifically with the frum communities. It is with the security forces, who very likely might do thigns entailing chilul shabbos when preparing or the large crowds up north or even to deal with safety issues at local bonfires even with smaller crowds.

In addition, if we light the bonfires and celebrate on Motzei Shabbos, other communities will as well, and perhaps they will not be quite as diligent about making sure all preparations are being done only after Shabbos.

To that effect, I remember one time I had to be in the hospital on Shabbos. The rabbi of the hospital spoke in the shul on Shabbos morning for a few minutes. After his dvar torah he also gave some instructions about using the shul over the course of the Shabbos. I remember he instructed the crowd to not make any ad hoc minyanim for maariv at an earlier time than that posted on the hospital schedule. he said that as soon as "borchu" is said in the hospital synagogue, the rest of the hospital goes back to normal operation. If someone arranges a maariv minyan at an earlier time than the official time, they could possibly be causing chilul shabbos, because other people around the hospital will act thinking Shabbos is over.

The message with Lag b'Omer is similar. if we frum communities light on Motzei Shabbos, others will think it is perfectly ok, and they won't take the precautions that we will take to guard the sanctity of Shabbos. There is both the issue of areivim, in that we must be concerned about the chilul Shabbos of others that we can help prevent (similar to the way we construct eiruv's even in cities and communities of non-religious people - to minimize their potential chilul Shabbos), as well as the issue of being indirectly responsible for their chilul Shabbos, because they thought it was ok when they realized that we were doing it on Motzei Shabbos.